D&D 5E "Unstoppable" new reaction introduced in Descent?

[Salthorae]

The Death's Head of Bhaal has the following reaction:
Unstoppable (3/Day): the death's head reduces the damage it takes from an attack to 0.

Has this ever been in a stat block before? It is nasty--almost resulted in TPK last night. Depending on the circumstance, more powerful than legendary resistance. It made for a great, challenging encounter.

There is an Epic Boon that does the same thing 1/Short Rest. I'd say it's pretty powerful, but not out of line with legendary resistances.

It would certainly annoy the player of the paladin who just smited their last spell slot on a critical to do 100 points of radiant daamge, only to hear the GM say, "The creature takes no damage."

Hah, I wouldn't say it like that either!

 

[Paul Farquhar]

@Nebulous merely stated he missed how it used to state how many appeared, which 1st ed did. I didnt infer he was speaking of balanced encounters, but I may have missed that.

You maybe right. I took them to be referring to something that was actually useful.