AD&D 1E XP Value for Monsters?

[Lanefan]

Ah, yes, NPC parties, or, to quote DMG, p. 21: “Men are the worst monsters”.

Challenge-wise, the xp values of NPC parties can definitely seem to be on the low side. But I’d say that is more a matter of “feel” and how each DM wants to award his PCs, for the xp values of each won't differ that much from those of monsters of similar hit dice if one would strictly stick to the EXPERIENCE POINTS VALUE FOR MONSTERS table in DMG, p. 85 (henceforth “XPV table”).

I’ve had to generate quite a few randomly generated NPC parties in the last couple of years, which has made me decide to use a number of basic rules.

First, I allocate xp values for “what could have been”—i.e., potential—rather than for what actually happened. This means that defeating the assassin before he has had a chance to assassinate one of the PCs with his poisoned dagger gets the party xp for “dealing with a threat that could have killed them” and bravo for that. I suppose this is rather in line with a red dragon's xp value simply including its breath weapon.

Second, in much the same way, I do not make a distinction between “spell use” and “use of minor (basically defensive) spells” for NPC spell casters. I roll for what spells the magic-user and iillusionist have in their spell books and then roll for the spells all NPC spell casters have memorized for that day. While that means that a magic-user can have magic missile and fireball in his spell book but only learned mending and water breathing in their stead that day, it still means that the PCs have dealt with one who “could have done them some serious harm if the dice hadn't spoiled things”. Also, I want to be on the safe side in case an encounter leads to multiple confrontations.
As to the extremely rare case that an NPC has psionics, I typically go for an EAXPA. But that is me not really understanding how psionics work and otherwise handling them differently for reasons.

Third, I do award xp for the PCs acquiring magic items, as I suppose is suggested in the rather splendid example of Gygaxiana under “EXPERIENCE VALUE OF TREASURE TAKEN” on p. 85 in the DMG. Also, you’d be surprised at the low number of magic items NPC parties have if one sticks to the tables on p. 176 in the DMG (and at the number of serpentine owls in existence). Also, this has the added advantage of NPC parties actually being worth a lot more than their xp values alone.

Fourth, I also use (a version of) the party levels vs monster levels calculation suggested by “ADJUSTMENT AND DIVISION OF EXPERIENCE POINTS” on p. 84 in the DMG. This means less xp for 6th-level PCs hacking their way through a 1st-level NPC party, and more xp for 1st-level PCs vs 6th-level NPCs.

I don't make the "fourth" adjustment. Xp values are what they are, regardless how easy or difficult the battle might be.

I just let the J-curve of advancement tables take care of characters gaining xp by mowing down mooks - they'll run out of mooks to kill long before they advance very far.

Fifth, I treat multi-classes and dual classes as “the highest level among them, plus the class abilities granted by each”.***

Sixth, I base NPC xp values on the “Treat (…) all levels as the n+1 category” rule.

So, in the examples as you gave them:

The specialized 6th-level fighter (225 + 8/hp; 125/175) would get a BXPV of 225, plus 1×EAXPA if the girdle and/or sword would mean that he can deal “maximum damage greater than 24 singly”, for a possible grand total of 400 + 8/hp.
Because experience has made me house-rule that fighters cannot hack their way through a tavern filled with commoners at a rate per round equal to their level, this also applies to this fighter being able to do the same to any men-at-arms accompanying the PCs, wherefore he doesn’t get an SAXPB for being able to attack four or more times per round. It’s a rule that dates back to ancient times, but I still like it so I’m not gonna change it.
Oh, and I don’t really know what you mean by “specialized”, but there you go.

Weapon specialization is a UA thing, giving Fighters to-hit and damage bonuses with a single weapon along with a faster-advancing attack-rate table with that same weapon. We use a toned-down version and it's become ingrained.

The 5th-level cleric/magic-user (150 + 6/hp; 75/125) would get a BXPV of 150, plus 1×EAXPA for “spell use”, plus 1×SAXPB for being resistant to sleep and charm spells because he is either a half-elf or an elf, plus 1×EAXPA if one of the blast wands would allow him to inflict “maximum damage greater than 24 singly”, for a possible grand total of 475 + 6/hp.
Also, if he is an elf, he would (arguably) get an SAXPB for having Intelligence 13+ (MM, p. 39), plus another SAXPB once I’ve figured out whether “surprise on” and/or being able to remain unseen is actually a special attack—or a special defense for that matter.
If he is a half-elf, the Monster Manual is vague on whether he is as intelligent as an elf, so I guess intelligence has to be rolled for.

The 7th-level assassin (375 + 10/hp; 175/275) would get a BXPV of 375, plus 1×EAXPA for assassination (“instant death without regard for hit points”), plus 1×SAXPB for missile discharge, plus 1×EAXPA for the poison, for a grand total of 1100 + 10/hp.
I’d say the backstab×3 doesn’t really come into play for the massive damage count. That is, unless you’d read “sword” (PHB, p. 27) as “any sword”, and would therefore allow him to backstab with a two-handed sword, in which case he would get another EAXPA for being able to inflict “maximum damage greater than 24 singly”.

Backstab at 7th level inflicts triple damage, so even with no bonus whatsoever a longsword can do 24 points at max. All it needs is a +1 bonus from anywhere to get to max 27 points and thus reach EAXPB criteria. And as it seems strange that it jumps from no bonus straight to EAXPB I'd be inclined to give SAXPB for the backstrike if the Assassin was using a longsword or similar without any bonus.

As for the C/MU, another houserule that's become so ingrained that I've forgotten the original is that anyone can multiclass if their stats allow it, unless the class is banned from multiclassing or the class is banned to the species.

As per the cleric/magic-user above, the jury is still out on whether being able to remain unseen/surprise is worth any xp.
Oh, and I see what u did there with your slippers of spider-climb.

The 6th-level illusionist (225 + 8/hp; 125/175) would get a BXPV of 225, plus 1×SAXPB for “high intelligence which actually affects combat”, plus 1×EAXPA for “spell use”, plus 1×EAXPA for psionics, for a grand total of 700 + 8/hp.

The 4th-level thief (90 + 5/hp; 40/75) would get a BXPV of 90, plus… nothing, for a grand total of 90 + 5/hp.
Again, I have not awarded xp for being able to remain unseen/surprise, as per the notes above.

Stealth and surprise give xp bonuses for creatures that have such e.g. Yeti thus IMO it makes sense it should apply here too.

***) I had a very specific reason for that, which I seem to have forgotten. So I’d be really interested to know where I can find the rule that says that a multi-classed character adds another +1 to its base xp value per additional class.

I took my cue on this from - and I forget where this is said but it's in there somewhere - the idea that when assigning levels to a party for purposes of determining whether a particular adventure is suitable for them, multiclass characters count as their highest level plus one per extra class provided the secondary classes aren't too far behind the leading class.

Thus, a C-5/MU-5 counts as a 6th. It makes sense that xp would work the same way.