Respawning Undead - Cave of Lamentation

[oldboot2023]

Any help appreciated, I'm in the Forest of Forgotten Spirits and I've used the two keys to get into the Cave of Lamentation. I immediately get attacked by two Rotwhisperer undead and defeat them, but they respawn...and respawn...and respawn. Am I missing something? I've tried to interact with the cave and there doesn't seem to be any way to make it stop?

[[TUS] Maggot]

Sadly I can't add anything helpful, but I've got the same problem. Attacked the two Undead just a moment ago and got butchered because they're invincible. What did I miss? Do I have to kill one or the other two forest spirits? Can the Undead only be defeated at night (I attacked around 15:00)?

Edit:
I left the map, did a little Crusading-stuff, let my group sleep until midnight and was able to defeat the Undead. The Pale Rotwhisper (the one with levels in Oracle) did return once, the Pale Marrognaw died like the well-behaved opponent he is on first try.
Maybe the time of day was the deciding factor, I don't know.

[Sangestik]

They both have an ability that causes them to respawn at the start of a round if their brother is still alive. They have to both be dead before a round ends.

[Winter Wolf]

Considering they each have spell casting abilities, 300+hp and high AC this is probably a no go unless you are maxed level. I have found it impossible.

[ReyCV]

It took me more than ten attempts. Just try and try again.