The army of Lanka, or like a third of it:
It's not just chaff tho, there's plenty of dangerous sacred E9N9 Assrapists and various other sacreds mixed in, all rushing in with mass flight on turn one (perhaps sending just the sacreds in on turn 1 would be better, but soulless are undisciplined). There's also a large thunderstrike communion and several very strong casters (Mandeha, pretender, D6 Heliophagus).
My force, surrounded:
Surprisingly, I didn't mess up my positioning and my casters were actually almost completely protected from the flying undead on turn 1.
Lankan damage on turn 1 from the flying alpha-strike was low since the undead chaff took up most of the room and they don't do all that much.
Here's the situation after the Undead Mastery cast on turn 1 (Lich with Crystal Matrix, Eye of the Void, and Spell Focus with 4 matrixed communion slaves):
Judging from post-battle numbers, about half of the soulless horde was controlled (3.5k). There was a second Undead Mastery cast supposed to happen, but the guy cast Skeletal Legion instead, dunno why. But Army of Lead is conveniently cast after Undead Mastery, affecting all the turncoat undead and making them tougher than their former compatriots that had Mass Protection.
All the newly-acquired undead provide the perfect distraction for Lanka's casters, who spend their time thunderstriking their own fickle minions:
After a couple of turns the horde thins rapidly and HP routs. I had some spells scripted, Cleansing Water and such, but I think the Magma Children did the most damage with their fire attacks and Fire Shield (against fire vulnerability from Mass Protection):
Final tally:
Looks like I earned myself 3.4k undead :D