Hmm, I'm having second thoughts about the Cha/Will. If it actually acts like a "poison" it could use a Con-based Fort save.
Also, upon reflection, if we made creatures with immunity to poison immune to the Slave Venom it means that Ythog-Nthlei would not be able to dominate Oozes or Plants, which doesn't seem right. How about we make them more resistant to the venom?
Anyhow, let's rough something out...
Slave Venom (Su): Any living creature hit by Ythog-Nthlei's bite attack must succeed at a DC 32 Fortitude save or be dominated by the Dark King for 1d10 minutes. The domination functions like the dominate monster spell. This supernatural venom can affect creatures immune to mind-affecting attacks or poison, but such immunity grants the victim a +4 bonus on their save (+8 if they're immune to both poison and mind-affecting effects, such as a Plant or Ooze). Creatures with saving throw bonuses against poison or compulsion effects (such as a dwarf's +2 against poison or an elf's +2 against enchantments) apply these bonuses when saving against Slave Venom. Slave Venom has no effect on nonliving creatures such as Constructs and Undead. Its save DC is Charisma-based.