Boss dealing damage before spawn?

https://medal.tv/games/path-of-exile-2/clips/ldSR10oNEvEkzidOY?invite=cr-MSxDUVAsMzc1MzkzNzM5&v=12

I happened to get one shot after activating a random monolith on maps. I havent ever been one shot on this character before, so I was really surprised.

After reviewing the footage I believe the boss was able to perform is high damage slam attack while spawning, not giving any waning of doing so. I can't think of any other scenario I'd take that kind of damage so suddenly.