Referring to lair actions. Most creatures cannot take the same Lair Action twice in a row:
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However, Strahd is an exception:
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Ah. Okay. I was looking more at the general rule on how lair actions work and didn't see anything. I didn't notice how each creature can sometimes have specific restrictions on lair actions like the Sphinx who apparently can only ever use each one only once between rests.
Strahd's lair actions is what makes him nearly impossible to kill when played like I think he should play: to win.
Example. Barbarian, Fighter (has sunsword), Cleric (has symbol), Rogue, & Monk against Strahd in his treasure room. Monk jumps in and hits a bit. Strahd phases through wall. Do not know where he will come out. Some vampire spawn are approaching so turn to face them to avoid being flanked. Strahd phases in, drops fireball in far room to hit several party members who had gone to face vampire spawn. Cleric ends up turning all spawn and they all scatter. Re-focus Strahd. Small hits get done, hurts what hit him back. More party members close in, he phases out. Spread out a bit not sure where he will return from. Several rounds later of waiting and it turns out he has run around the hallways and come back in through the study having regened any HP he lost earlier while party has depleted in HP.
So this back and forth keeps happening. Try to catch up to him, he closes the doors to separate party to divides and conquers. Could the party stick closer together? Maybe. But turn orders don't always allow for it. And then party might chase into a long hallway, he appears from nowhere and just fireballs again anyway so grouping up isn't going to help either. That is just playing smart with the resources he has. No minions really faced at all at this point. This is just him.
Now a different party could have an easier time. A warlock with eldritch blast, a wizard with counterspell, etc. But I can easily see ways you can have Strahd work around that too based on just the first two lair actions. And you won't necessarily have the perfect spells ready to go unless you somehow have perfect knowledge of how Strahd can fight and what he can do which people who haven't played the campaign before won't have knowledge of.
I'm working on changing his abilities now for my campaign. I just don't want to make the players feel powerless because of a stupidly strong lair action. Either I take away the lair action to replace it with a custom spell (like stoneshape but 5th level for that phasing effect) so therefore a limited amount of uses, or I keep it in as the cleric player who died has come back with a phase-like ability that has limited uses (costs a hit die per use) that could chase him through the walls with the holy symbol. Or I could take it out completely and come up with something entirely different. I'm more leaning towards the spell idea. He has spent centuries in Barovia and has studied magic. Why wouldn't he have invented his own spells? My narrative is also slightly different and the campaign expanded. I took things from the old domains of dread material and brought it in from the beginning, and as it turns out they are now coming out with a new ravenloft book. Hah.