The party’s Cleric and Wizard have worked together to summon and bind a Unicorn to their side for a year and a day. Now they get to enjoy an intelligent mount with legendary actions, teleportation, and extra healing. It can even have a lair of its own.
The party has gone inside a Lich’s domain. Is there anything the party can have the Unicorn do so that it can establish this very same location as its own domain, such that it becomes the lair of both the Lich and the Unicorn? If so, what does that process look like?
As an important note, this particular Lich is very pointedly not interfering with the party’s preparations up until they meet. This campaign is more of a hack and slash, so there’s not really any deeper underlying reason as to why. What is important is that it doesn’t violate the rules.
Here’s the subcomponents of this question, as I see it:
Can any creature turn any location into its lair?
Can more than one creature treat the same location as their lair?
What is the process for a creature turning an area into its lair?
I believe the answers to (1) and (2) are yes because there isn’t a specific rule that I know of that forbids this, so the main question here is (3). However, if you can cite a rule that shows the answer to (1) or (2) is a “no,” I believe that in this scenario it renders the succeeding questions moot, so that’s also an acceptable answer.