My players are on their way to an underground, fallen kingdom to pursue the spellbook of a very powerful wizard in order to destroy it and make sure the magics within will never fall into the hands of mortals again. There's a problem, though: the wizard it belonged to is still there, except now he's a spooky scary Lich trapped by ancient Dwarven craft and can't leave the place.
How do I make a boss encounter where this Lich, who is a 20th level Wizard encounters a party of 7th level characters without it ending in a TPK?
They don't necessarily have to fight, or rather, shouldn't fight it. I just want them to see what they're up against, maybe have a scene where they madly escape the Lich's lair while it pursues them.
There are a few things that should happen:
>Players get the spellbook they were coming to get
>The players stop the Lich's henchmen from collecting dragonbones (the Lich wants to transfer his consciousness into a dragon skeleton to escape his prison/lair)
>The players get a genuinely terrifying encounter with a far superior foe.
Now, the problem is, how do I make it so that
>Players understand they need to gtfo and they just can't win this fight
>They get an enjoyable experience from escaping
>It doesn't end in a TPK
>Back-up plan in case they go full retard to give them even a small chance of not winning, but coming out of it alive?
Noteworthy is that one of the players has an amulet that stops death effects when wearing it (it has a certain number of charges), so I was gonna have the Lich start the encounter with a Power Word: Kill on her, have it bounce off and that way give them at least one round to do something smart.
Any ideas? Especially on how to make escaping a superior foe itself an encounter with some swashbuckling action and spookiness.