El Mahdi
Muad'Dib of the Anauroch
I'm not aware that the Grey Mouser ever casts a spell that's not from a scroll (which is part of why AD&D thieves can use scrolls). In 5e that's a feat. In AD&D that's a high level thief ability. And I think that outside his origin story the Grey Mouser casts a grand total of one spell in total.
Why would "outside of his origin story" matter? It's part of who he is. In his origin story he displays arcane knowledge (items, potions, etc.), spellcasting, using rituals, and even necromancy. All of which would not simply be covered by a feat. At a minimum it would require one level of Wizard. Incidentally, the first couple of levels of 5E are meant to be more "novice" characters; perfectly describing the Apprentice level of the Mouser.
He pulled the odd trick, yes. The one I recall was grounding out his weapon with a wire and deflecting spells with it. He might have just had Arcana and improvised now and then. Then again, his time as a Wizard's Apprentice before becoming a Rogue could be handled by a Background (in 4e there was even a Wizard's Apprentice 'theme'). In 1e, he was just a Thief - well, not 'just.' Arguably, the archetype for the D&D Thief, which, like the Mouser, favored the Sling over the bow, and had a facility for translating languages (Read Languages % 'Special' Ability), which extended to scrolls, corresponding to Grey Mouser's dabbling in magic in his early years.
Much more than just the "odd trick"; and a background would not provide all that the Mouser knows and can do.
I think far too often, many get their ideas of Fafhrd and the GrayMouser not from reading the books themselves, but from second hand sources(essays about the characters, homage stories, and unfortunately from D&D itself). Or at least, those secondary sources somehowtake on supremacy in their informing of the characters, rather than theoriginal stories.
It’s funny how our memories can be so malleable and oft suspect.
While the Mouser obviously favors a martial response and relying on hismartial skills in most situations, his arcane knowledge and ability is bothimplicit and explicit throughout the books. Outside of his origin story, his use of magic is quite rare, but I think that's more of a choice - having crossed the line into very dark magics - than it is his inability to use them.
In the Mousers origin story (TheUnholy Grail – 1962), there is this:
Janarrl scrambled to his feet, eyed his daughter andthe boar. Then his gaze traveled slowlyaround the glade, full circle.
Glavas Rho’s apprentice [the Mouser] was gone. “North be south, east be west. Copse be glade and gully crest. Dizziness all paths invest. Leaves and grasses, do the rest.”
Mouse mumbled the chant through swollen lips almost asthough he were talking into the ground on which he lay. His fingers arranging themselves into cabalisticsymbols, he thumbed a pinch of green powder from a tiny pouch and tossed itinto the air with a wrist-flick that made him wince. “Know it, hound, you are wolf-born, enemy towhip and horn. Horse, think of theunicorn, uncaught since the primal morn. Weave off from me, by the Norn!”
The charm complete, he lay still and the pains in hisbruised flesh and bones became more bearable. He listened to the sounds of thehunt trail off in the distance.
That was a spell, not a scroll - complete with verbal, somatic, and materialcomponents – a spell that the Mouser used to hide himself from the murderers ofhis master, after he had tried to kill the Duke responsible and was beatenwithin an inch of his life for the attempt.
But then he goes on and crosses the line to Necromancy (and the section also implies the arcane knowledge he possesses):
His face waspushed close to a patch of grass. He sawan ant laboriously climb a blade, fall to the ground, and then continue on itsway. For a moment he felt a bond ofkinship between himself and the tiny insect. He remembered the black boar whose unexpected charge had given him achance to escape and for a strange moment his mind linked it with the ant.
Vaguely he thoughtof the pirates who had threatened his life in the west. But their gay ruthlessness had been adifferent thing from the premeditated and the presavored brutality of Janarrl’shuntsmen.
Gradually angerand hate began to swirl in him. He sawthe gods of Glavas Rho, their formerly serene faces white and sneering. He heard the words of the old incantations,but they twanged with a new meaning. Then these visions receded, and he saw only a whirl of grinning facesand cruel hands. Somewhere in it thewhite, guilt-stricken face of a girl. Swords, sticks, whips. Allspinning. And at the center, like thehub of a wheel on which men are broken, the thick strong form of the Duke.
What was theteaching of Glavas Rho to that wheel? Ithad rolled over him and crushed him. What was white magic to Janarrl and his henchmen? Only a priceless parchment to bebesmirched. Magic gems to be trampled infilth. Thoughts of deep wisdom to bepulped with their encasing brain.
But there was theother magic. The magic Glavas Rho hadforbidden, sometimes smilingly but always with an underlying seriousness. The magic Mouse had learned of only by hintsand warnings. The magic which stemmedfrom death and hate and pain and decay, which dealt in poisons andnight-shrieks, which trickled down from the black spaces between the stars,which, as Janarrl himself had said, cursed in the dark behind the back.
It was as if allMouse’s former knowledge – of small creatures and stars and beneficialsorceries and Nature’s codes of courtesy – burned in one swift suddenholocaust. And the black ashes took lifeand began to stir, and from them crept a host of night shapes, resembling thosewhich had been burned, but all distorted. Creeping, skulking, scurrying shapes. Heartless, all hate and terror, but as lovely to look on as blackspiders swinging along their geometrical webs.
To sound a huntinghorn for that pack! To set them on thetrack of Janarrl!
Deepin his brain an evil voice began to whisper, “The Duke must die. The Duke must die.” And he knew that he would always hear thatvoice, until its purpose was fulfilled.
Later, he uses necromantic sympathy magic and ritual magic to attack and kill the Duke:
Come nightfall onthe seventh day, when dinner was being served in the great banquet hall, withmuch loud talk and crunching of rushes and clashing of silver plates, Janarrlstifled a cry of pain and clapped his hand to his heart.
“It is nothing,”he said a moment later to the thin-faced henchman sitting at his side. “Give me a cup of wine! That will stop it twinging.”
But he continuedto look pale and ill at ease, and he ate little of the meat that was served upin great smoking slices. His eyes keptroving about the table, finally settling on his daughter.
“Stop staring atme in that gloomy way, girl!” he called. “One would think that you had poisoned my wine and were watching to seegreen spots come out on me. Or red onesedged with black, belike.”
This brought ageneral guffaw of laughter which seemed to please the Duke, for he tore off thewing of a fowl and gnawed at it hungrily, but the next moment he gave anothersudden cry of pain, louder than the first, staggered to his feet, clawedconvulsively at his chest, and then pitched over on the table, where he laygroaning and writhing in his pain.
“The Duke isstricken,” the thin-faced henchman announced quite unnecessarily and yet most portentouslyafter bending over him. “Carry him tobed. One of you loosen his shirt. He gasps for air.”
…
Ivrian dismountedand moved on. The forest wasportentously quiet, as if all animals and birds – even the insects – had gone. The darkness ahead was almost tangible, as ifbuilt of black bricks just beyond her hand.
Then Ivrian becameaware of the green glow, vague and faint at first as the ghosts of an aurora. Gradually it grew brighter and acquired aflickering quality, as the leafy curtains between her and it became fewer. Suddenly she found herself staring directlyat it – a thick, heavy, soot-edge flame that writhed instead of danced. If green slime could be transmuted to fire,it would have that look. It burned inthe mouth of a shallow cavern.
Then, beside theflame, she saw the face of the apprentice of Glavas Rho, and in that instant anagony of horror and sympathy tore at her mind.
The face seemedinhuman – more a green mask of torment than anything alive. The cheeks were drawn in; the eyes wereunnaturally wild; it was very pale, and dripping with cold sweat induced byintense inward effort. There was muchsuffering in it, but also much power – power to control the thick twistingshadows that seemed to crowd around the green flame, power to master the forcesof hate that were being marshaled. Atregular intervals the cracked lips moved and the arms and hands made setgestures.
It seemed to Ivrianthat she heard the mellow voice of Glavas Rho repeating a statement he had oncemade to Mouse and to her. “None can useblack magic without straining the soul to the uttermost – and staining it intothe bargain. None can inflict sufferingwithout enduring the same. None can senddeath by spells and sorcery without walking on the brink of death’s own abyss,aye, and dripping his own blood into it. The forces black magic evokes are like two-edged poisoned swords withgrips studded with scorpion stings. Onlya strong man, leather-handed, in whom hate and evil are very powerful, canwield them, and he only for a space.”
In Mouse’s faceIvrian saw the living example of those words
…
…the thin-facedhenchman was already sniffing around in the cavern’s mouth like a well-trainedferret. He gave a cry of satisfactionand lifted down a little figure from a ledge above the fire, which he nextstamped out. He carried the figure asgingerly as if it were made of cobweb. As he passed by her, Ivrian saw that it was a clay doll wide as it wastall and dressed in brown and yellow leaves, and that its features were a grotesquecopy of her father’s [Janarrl]. It was piercedin several places by long bone needles.
“This is thething, oh Master,” said Giscorl, holding it up, but the Duke only repeated, “Quick,Giscorl!” The henchman started towithdraw the largest needle which pierced the doll’s middle, but the Dukegasped in agony and cried, “Forget not the balm!” Wherupon Giscorl uncorked with his teeth andpoured a large vial of syrupy liquid over the doll’s body and the Duke sighed alittle with relief. Then Giscorl verycarefully withdrew the needles, one by one, and as each needle was withdrawnthe Dukes breath whistled and he clapped his hand to his shoulder or thigh, asif it were from his own body that the needles were being drawn. After the last one was out, hje sat slumpedin his saddle for a long time. When hefinally looked up the transformation that had taken place was astonishing. There was color in his face, and the lines ofpain had vanished, and his voice was loud and ringing.
“Take the prisonerback to our stronghold to await our judgment,” he cried.
…
A narrowing of thepath brought her close beside him. Shesaid hurriedly, ashamedly, “If there is anything I can do so that you willforgive me a little…”
The glance he benton her, looking sidewise up, was sharp, appraising, and surprisingly alive.
“Perhaps you can,”he murmured softly, so the huntsmen ahead might not hear. “As you must know, your father will have metortured to death. You will be asked towatch it. Do just that. Keep your eyes riveted on mine the wholetime. Sit close beside your father. Keep you hand on his arm. Aye, kiss him too. Above all, show no sign of fright orrevulsion. Be like a statue carved ofmarble. Watch to the end. One other thing – wear, if you can, a gown ofyour mother’s or if not a gown, then some article of her clothing.” He smiled at her thinly. “Do this and I will at least have the consolationof watching you flinch – and flinch – and flinch!”
…
[on the rack] With startling suddenness the pain returned andincreased. The twinges became needlestabs – a cunning prying at his insides – fingers of force crawling up his armsand legs toward his spine – an unsettling at the hips. He [Mouser] desperately tensed his musclesagainst them.
…
Footsteps brokethat scene, as stones destroy a reflection in water, and brought the presentback. Then a voice: “Your daughtercomes, oh Duke.”
…
It meant addedpain for him to turn his head so he could see the doorway: yet he did so,watching her figure define itself as it entered the region of ruddy light castby the torches.
Then he saw theeyes. They were wide and staring. They were fixed straight on him. And they did not turn away. The face was pale, calm with a deadlyserenity.
He saw she wasdressed in a gown of dark red, cut low in the bosom and with slashes inset withyellow silk. (her mother’s dress)
And then the soulof Mouse exulted, for he knew that she had done what he had bidden her. Glavas Rho had said, “The sufferer can hurlhis suffering back upon his oppressor, if only his oppressor can be tempted toopen a channel for his hate.” Now therewas a channel open for him, leading to Janarrl’s inmost being.
Hungrily, Mousefastened his gaze on Ivrian’s unblinking eyes, as if they were pools of blackmagic in a cold moon. Those eyes, heknew, could receive what he could give.
He saw her searherself by the Duke. He saw the Dukepeer sidewise at his daughter and start up as if she were a ghost. But Ivrian did not look toward him, only herhand stole out and fastened on his wrist, and the Duke sank shuddering backinto his chair.
“Proceed!” heheard the Duke call out to the torturer, and this time the panic in the Duke’svoice was very close to the surface.
The wheelturned. Mouse heard himself groanpiteously. But there was something inhim now that could ride on top of the pain and that had no part in thegroan. He felt that there was a pathbetween his eyes and Ivrian’s – a rock-walled channel through which the forcesof human spirit and of more than human spirit could be sent roaring like amountain torrent. And still she did notturn away. No expression crossed herface when he groaned, only her eyes seemed to darken as she grew still more pale. Mouse sensed a shifting of feelings in hisbody. Through the scalding waters ofpain, his hate rose to the surface, rode atop too. He pushed his hate down the rock-walledchannel, saw Iviran’s face grow more deathlike as it struck her, saw hertighten her grip on her father’s wrist, sensed the trembling that her father nolonger could master.
The wheelturned. From far off Mouse heard asteady, hear=tearing whimpering. But apart of him was outside the room now – high, he felt, in the frosty emptinessabove the world. He saw spread out belowhim a nighted panorama of wooded hills and valleys. Near the summit of one hill was a tight clumpof tiny stone towers. But as if he wereendowed with a magical vulture’s eye, he could see through the walls and roofsof those towers into the very foundations beneath, into a tiny murky room inwhich men tinier than insects clustered and cowered together. Some were working at a mechanism whichinflicted pain on a creature that might have been a bleached and writhingant. And the pain of that creature,whose tiny thin cries he could faintly hear, had a strange effect on him atthis height, strengthening his inward powers and tearing away a veil from hiseyes – a veil that had hitherto hidden a whole black universe.
For he began tohear about him a mighty murmuring. Thefrigid darkness was beaten by wings of stone. The steely light of the stars cut into his brain like painless knives. He felt a wild black whirlpool of evil, likea torrent of black tigers, blast down upon him from above, and he knew that itwas his to control. He let it surgethrough his body and then hurled it down the unbroken path that led to twopoints of darkness in the tiny room below – the two staring eyes of Ivrian,daughter of Duke Janarrl. He saw theblack of the whirlwind’s heart spread on her face like an inkblot, seep downher white arms and dye her fingers. Hesaw her hand tighten convulsively on her father’s arm. He saw her reach her other hand toward theDuke and lift her open lips to his cheek.
Then, for onemoment while the torch flames whipped low and blue in a physical wind thatseemed to blow through the mortised stones of the buried chamber…for one momentwhile the torturers and guards dropped the tools of their trades…for oneindelible moment of hate fulfilled and revenge accomplished, Mouse saw thestrong, square face of Duke Janarrl shake in the agitation of ultimate terror,the features twisted like heavy cloth wrung between invisible hands, thencrumpled in defeat and death.
There are other references of Mouser using magic, though I can’t find the specificsections now. I vaguely remember one though,where the Mouser uses a divination ritual to seek information or guidance onwhat he is to do next.