Wtf boss damage

i’ve seen some very weird boss fights from people using thorns build, they almost don’t do any dmg to the boss then suddenly one shots him
how?
that’s an example www.youtube.com/watch?v=ovUWwWYXqKs

It’s Fell Soothsayer’s aspect proc.

From Maxroll, click the Pit Push variant and read the disclaimer.
https://maxroll.gg/d4/build-guides/balazan-quill-spiritborn-guide#build-variants-header