Setup: PCs are 8th level, a caster-heavy group drained of resources, in a battle with an "atrophied" lich (has some lair and legendary lich abilities, capped as 11th level caster, reduced DCs, no phylactery so it won't stick around if there's a chance it could be destroyed) ...
IF the battle keeps going as-is, it's a TPK. We paused in battle between session, so I have till next weekend to regroup.
It Got Out of Control When... The atrophied lich used its Lair tether ability (CON save or take half the damage the lich takes) and the Shield Guardian ability, combined with legendary actions to repeatedly cast Toll the Dead. I felt it's a fair tactic a genius-level bad guy would use, especially since he's been scrying the PCs for weeks now.
But, it's still too much... The Guardian is now down, but the atrophied lich is still renewing spells each round, doing great on HP, and tethering to the PC wizard. He's been scrying them for weeks now, so I've tailored his strategies to defeat their abilities. The PC fighter is on fumes. The PCs have exhausted their powers trying to keep her afloat.
So, damn, what do you do? I believe now the PCs should have been 9 or 10th level before this battle, my bad on that part. Too late now to correct. It's game on and if I play the Boss intelligently, it knows it's winning and simply has to spam Toll the Dead until everyone is dead. Looking for ways to throw in some (believable) chances for the PCs rather than mercilessly mow them down. Also, on a tangent, may approach the group again about limiting the # of attack cantrips one can cast between short rests. The fighter PC is probably not happy with cantrip spam at the moment...
I underlined what stood out to me.
Single higher level foe, with guard, versus a lower level party. Seems reasonable at first blush. A high level foe should be prepared. But, scrying for weeks? I presume that this is part of the convoluted backstory, but I am a little surprised the party was this unprepared.
The lich doesn't mitigate the damage it takes, but can inflict additional damage when it is harmed? Not sure what you mean by it is "renewing" spells each round. Are there limits to legendary, lair, or similar actions? I forget what is per round or encounter.
- Wizard / Cleric should cast
darkness,
minor illusion,
blindness, or something else to block line of site so that
toll the dead is useless. Or, do something that forces the lich to cast spells that the cleric and wizard counter. Leaving a lich at distance is a death sentence. (Do I understand correctly that it is doing 3d8 / 3d12 up to four times a round? That's a
cantrip? It seems that a save makes for no damage, but its save DC must be strong. That seems quite strong, but then it is not casting any other spells. But, then, it is casting three cantrips a round plus a spell.)
- Cleric should bombard the lich with
sacred flame. This needs line of sight, obviously, so it may conflict with the previous tactic. The cleric should be healing the fighter otherwise.
- Wizard should cast
dispel magic or
counterspell on the tether ability. (I don't know if that is strictly BTB, but it should be given the strength of this power.) Is this a lasting power or does it need to be renewed?
- Thief should be holding action to stab the lich when they cast. Waste the legendary actions. Also provide flanking advantage to the fighter.
- The fighter needs to be up front and personal with that lich, and the rest of the party needs to keep them alive. Hammer it into the wall like John Henry's last stand. Gain bonus attacks whenever it tries to cast a spell.
- The fighter can tackle the lich, yell "save yourselves", and die a heroic-ish death. Or, wait a minute, can casters cast when grappled? Have the fighter grapple, cleric heal the fighter, and everyone else pound the snot out of it?
As to what the DM can do, and actually answer the OP's question:
- The cleric / paladin / warlock can ask their god / inspiration / patron for a miraculous teleport away.
- With the next successful hit that does >10 hp damage the lich teleports away. "Curse you meddlesome kids!"
- With the next AoE the PCs cast part of the ceiling collapses between the adversaries.
- Secure in its power, the lich allows the PCs to escape when they flee. They obviously aren't a threat, but it is going to find a new lair.
