Help finding a monster!

[Invader]

There's an undead creature lower than CR 7 that when there's more than one in a group they share a HP pool but I can't for the life of me remember what it is. Can anyone help me out?

Edit* It may not be an undead monster so if there are any other low level creatures that have the ability to share a HP pool it still might be it.

[denthor]

There is an undead wall but I forget the CR.

This is does not move. This mostly a wailing wall that only attacks when someone else is near and alerts the living.

[Invader]

Nope it was def more of just a regular undead monster.

[Lateral]

You might be thinking of the Legion Devil (Fiendish Codex II).

[Invader]

You might be thinking of the Legion Devil (Fiendish Codex II).

No I know Legion Devils because I'm dying to use them a little later in the campaign lol. This was something different.