When do the skeletons, zombies, or ghouls created by Orcus' lair action take their turns in initiative?

Question

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.

Answer 1

It's more or less up to the DM to decide how to handle this, but typically when new monsters enter a fight, such as when a new wave of enemies arrives on the scene of the fight, they just roll their initiative as normal and get slotted into the order wherever their number falls. That's what I'd do with Orcus -- just roll initiative for the undead he created, and they go when their turn comes up.

When I say "roll initiative as normal," I mean do it however you as DM would normally handle initiative for a group of monsters in a fight. When I'm DMing, I usually just have all the monsters with the same stat block roll together and act on the same initiative -- "5 gnolls" or "7 bandits" or whatever. Some DMs like to roll every creature individually but I don't find that brings much benefit to the game in exchange for the complexity it adds. (The exception is if I have 2 or 3 enemies who are meant to collectively be the centerpiece of the fight. In that case, each one gets a separate initiative roll and a unique name or description. So my players might see three ogres named Bert, Bill, and Tom, or a pair of white dragons referred to as One-Eye and Broken-Horn.)

So to keep things simple, if it were me, whenever Orcus uses this power, I'd pick one monster type (skeleton, zombie, or ghoul), create up to six of them, roll their collective initiative once, and slot them into the order wherever they go.