Mouser cast a Death Spell from himself through his girlfriend, who was holding her father's wrist while said father was making her watch Mouser being tortured to death (daddy hated mages, and Mouser's first apprenticeship was under a magic-user). So he cast a Death Spell into her, which she then channeled into her father.
A little better than a 1st level magic-user!
Since there seems to be some interest, here's the rundown on the Mouser:
GRAY MOUSER (hero)
ARMOR CLASS: 2
MOVE: 12"
HIT POINTS: 96
NO. OF ATTACKS: 3/2 (or 3; see below)
DAMAGE/ATTACK: By weapon type
SPECIAL ATTACKS: Nil
SPECIAL DEFENSE: Nil
MAGIC RESISTANCE: Standard
SIZE: M
ALIGNMENT: Neutral
CLERIC/DRUID: Nil
FIGHTER: 11th level fighter
MAGIC-USER/ILLUSIONIST: 3rd level magic-user
THIEF/ASSASSIN: 15th level thief
MONK/BARD: Nil
PSIONIC ABILITY: Nil
Attack/Defense Modes: Nil
S: 16 (0, +1) I: 18 W: 14 D: 19 C: 17 CH: 18
This short man of the western cities supplies the refinement and delicacy his brawny partner lacks. He calls his rapier Scalpel and the dagger he uses (balanced for throwing) Cat's Claw. The Gray Mouser is very fond of studying things arcane, but has not the magical skills to master his studies. He has a great aversion to anything sharp being pointed at him, an aversion that has saved his life many times. He generally fights with Scalpel in his right hand and Cat's Claw in his left, incurring no penalties to hit due to his phenomenal dexterity. When so armed, he strikes 3 times per round, with a 50% chance that the third strike will be with either weapon.
While the Mouser grew up on the streets (with all that that implies), his teen years were spent with an old hedge magician who taught him the ways of law and a little magic. It was these days spent in the forests with his mentor that gave Mouser his streak of morality. It was shortly after this that the young man met his lifelong friend Fafhrd. While the Gray Mouser is a thief, a liar, and a trickster supreme, he can be rlied upon to keep faith for causes he supports and people he favors.
...
Fritz Leiber wrote a lot of stuff for early issues of THE DRAGON, so the liberties taken by statting up the Nehwon mythos in DEITIES & DEMI-GODS can be forgiven I reckon. There's a lot of rule bending - no, breaking - going on in those stats but as an NPC and an iconic one there's a lot of DM handwaving if you're gaming in Lanhkmar.
The magic of the stories of Fafhrd & The Gray Mouser seem to fall between three axes: low-level sorts of cantrips (by AD&D standards) that nonetheless are hard to pull off, insanely high-level ritual magic that requires a lot of prep time that nobody but arch-wizards can pull off (there's a whole story with Faf & Mou going after a wizard in Damascus - yes, that one - who is doing the whole lich-with-phylactary trick), and the outright alien stuff that nobody knows how/is handwaved by the author that Sheelba and Ningabule regularly perform.
Spoiler alert:
There's a cult of witches in Rime Isle (young girls, and all apparently white witches) who, in "The Mouser Goes Below" show a few cantrips, etc.
I know there was a LANKHMAR boxed set released, but were I to run an AD&D game set in Nehwon, I'd likely gin up something which, while using a lot of the creatures and so on in DEITIES & DEMI-GODS would revamp the magic system at the very least...
Sorry, I wandered a bit. Hope this was informative.