Orcus' second lair action reads
Orcus causes up to six corpses within the lair to rise as skeletons, zombies, or ghouls. These undead obey his telepathic commands, which can reach anywhere in the lair (MTF 153, OoTA 245).
It seems like a fairly straightforward summon effect at first. But when do they act? There are a few possibilities, pulled from similar mechanics, none of which match the text above.
Compare to the spell conjure animals (PHB 225) and similar PHB conjure-creature spells, whose relevant text reads:
The summoned creatures are friendly to you and your companions. Roll initiative for the summoned creatures as a group, which has its own turns.
We also have the summon fiend spell (TaCo 112) and the other summons from that book, which read:
The creature is an ally to you and your companions. In combat, the creature shares your initiative count, but it takes its turn immediately after yours.
It is also possible that since it's a lair action, the Orcus-summoned undead act on initiative count 20, but that is pure speculation. However, the other examples don't match his feature text either, and so are just as speculative.