The Index Reloaded --- (Index of the Giant's Comments II)

[Malimar]

But the strip says it was the ability rather than the spell, and we already know RC doesn't have the levels to use Command Undead against Xykon, because he is a mid-high cleric who just gained 9th level spells that day, rather than the unstoppably powerful 40th-level cleric that he would need to be to Command an epic-level lich.

Redcloak has to be twice the level of the undead in question. He is nowhere near twice the level of Durkon, and Xykon is higher. He just received 9th level spells, which means he's level 17. He'd need to be at least level 28 to use it against Durkon and 42 to use it against Xykon.

Not that it makes any difference to the matter at hand, but lichs have +4 turn resistance; a cleric would have to be at least 50 to command Xykon. Vampires have turn resistance, too, so at least 36 to command Durkula (Redcloak can, however, still rebuke Durkula, if he rolls well on his turning check).