So suppose a group of adventurers use a mirror of opposition to summon a army of evil duplicates, right in the middle of a drow spider festival. The 1st edition version of this item doesn't seem to have any limit to the number of duplicates it can summon, and it doesn't even require anyone to see the reflection in order to function. If you're reflected, you're duplicated! So presumably displaying it to a crowd would duplicate the entire crowd -- with murderous counterparts of themselves. How much experiences would the ensuing slaughter be worth?
First, note that the clones aren't particularly strategic. Each one wants only to mindlessly kill its counterpart. So the clones are certainly going to lose this fight, once the opposition gets organized. Let's assume that they can, at most, eliminate about 50% of those present, and that they'll always manage to kill at least 10%. Then we'll dice with d100 to see the exact fraction, keeping the first result between 10 and 50. And the drow get.... 11%! It's almost as if someone tipped them off to trouble. That's what comes of warning the heads of three major drow houses.
Now we need demographics of festival attendance. The city of Emru-Marol has 7000 families, but is part of a much larger realm (Emru-Marolvi) of more like 250,000 families. However, most of those "families" are slaves of the drow. The drow themselves are probably only about 5% of the total population of the realm. So we'll assume a base attendance of around 10,000, and assume that the other 2500 are staying home to keep the slaves in line. That seems like a sensible attendance figure for a major festival, given the physical dimensions of their temple.
All of these attenders are presumably leveled characters, since there are no "0th-level" adult elves. (We'll ignore children, since that's always an ethically uncomfortable subject, and just say they've stayed home. We'll also ignore the victims of their human sacrifices, since those are being brought out a few at a time.) The standard demographic level ratio for ACKS gives a distribution of something like
1 12th
1 10th
2 9th
5 8th
9 7th
20 6th
50 5th
120 4th
240 3rd
700 2nd
2000 1st
plus another 700 black widow spiders watching from above in hopes of snacking on a sacrifice victim. Here I've divided the ACKS numbers by 2 for all levels below 6th, since those soldiers are the ones staying home to suppress the slave revolts.
OK, now we need to check to see who lives. For all levels above 5th, I'm just going to roll randomly against the target of 11%, to see if they can beat it... and they all do, one after another! So no major drow leaders have been slain.
For lower-level characters, I'll just assume a flat ratio or 89% survivors. The 11% that die come out to...
2 6th x820 = 1640
5 5th x500 = 2500
13 4th x190 = 2470
25 3rd x80 = 2000
77 2nd x38 = 2926
220 1st x16 = 3520
77 spiders x80 = 6160
Total 21,216 xp
Divided over 8.5 shares (assuming 1.5 shares for each henchman) gives 2496 xp per party member, with the lizard henchmen getting a half-share of 1248 xp.
There were a few extra drow killed during the impromtu dance recital by Grimdark, but that's basically a rounding error compared to the list above. But since I'm a nice GM, I'll call it an extra 100 xp for anyone at the dance recital. That would be... Grimdark, Zigg, and someone else I forget. Will's one-eyed barbarian. I always forget him!
Oh, yes -- and to acquire the mirror in the first place, that took a win in the gladiatorial arena. So our two gladiators (Zigg and D) each get an extra
2 werewolves x135 = 270 xp
or an extra 135 xp each.