Small Party + Tempest Spine?

[Xanthrawl]

We made it all the way to the end only to discover that we needed three runes (we had found one by accident) in order to complete the puzzle and remove his invulnerability.

Oh well, maybe next time.

There are 3 Runes: One in Fire and Ice (between the 2 chests). Two is in the shrine room near the chest after the room with the beholder. And Three is in an alcove next to where you killed the Marut. It's easiest to make sure one person has all 3 runes, so you don't have to figure out which rune goes where. You just use all three and you're set.

[sirgog]

We made it all the way to the end only to discover that we needed three runes (we had found one by accident) in order to complete the puzzle and remove his invulnerability.

Oh well, maybe next time.

Yeah, that's the second or third of the 'sure to wipe a small party with no foreknowledge' things that was mentioned before. (The first is Fire and Ice; and you might consider one particular 'oh **** we just spawned a beholder during a big fight' moment to be another, although you may not trigger that and you may survive it anyway, it's not a certain wipe.