My previous experience with Lair Actions has been when the party fought Strahd (a vampire lord in Curse of Strahd) and Arauthator (an adult white dragon in Rise of Tiamat). Both of these creatures had multiple Lair Actions, but all of the actions either specified that they only applied to the creature (Strahd / Arauthator) itself or to a target it could see.
My party will soon be facing a mummy lord (for the particular mummy lord and circumstances of its lair, see this question, but be aware that even the question title contains spoilers). The mummy lord's lair actions are:
On initiative count 20 (losing initiative ties), the mummy lord takes a lair action to cause one of the following effects; the mummy lord can’t use the same effect two rounds in a row.
- Each undead creature in the lair can pinpoint the location of each living creature within 120 feet of it until initiative count 20 on the next round.
- Each undead in the lair has advantage on saving throws against effects that turn undead until initiative count 20 on the next round.
- Until initiative count 20 on the next round, any non-undead creature that tries to cast a spell of 4th level or lower in the mummy lord’s lair is wracked with pain. The creature can choose another action, but if it tries to cast the spell, it must make a DC 16 Constitution saving throw. On a failed save, it takes 1d6 necrotic damage per level of the spell, and the spell has no effect and is wasted.
Note that none of these Lair actions refer to the mummy lord itself or explicitly state that it needs to see the target of these effects.
This is particularly relevant to the specific mummy lord in question, as it does not leave its room, but its lair itself is large; players would have a chance to be affected by all three lair actions at some point during their exploration of the lair even while not in the physical presence of the mummy lord (if that is possible).
The answers to this question all state that a monster must be present in its lair in order to take lair actions, because it is the monster itself taking the actions, and I agree with that. This answer in particular states that since the Lair Actions happen on Initiative 20, and are taken by the monster, the monster must be part of the Initiative order, and I agree with that as well.
While it could be argued that a mummy lord that did not leave its room would not be part of a combat in another room, and thus would not be in the initiative order, I think that argument is circular. I could just as easily say that the Lair Actions themselves make the mummy lord part of a combat even if it is not physically present by providing it with actions that can affect the course of the combat, so that the mummy lord is placed in the initiative order of every combat in their lair even they are not physically present and even if their only action each round is to take their Lair Actions on Initiative 20.
Can monsters use lair actions on targets in any part of their lair, even when they are not in close proximity and even when they cannot see the targets?