how do i detect a players death

I'm making a mod where the worldborder plays a big role.

I'm trying to make it so that when the player dies it shrinks the worldborder by 5 blocks, however I'm having an issue where i can't find the right procedure to detect specificly the player died and then execute the correct command

i currently have is set up like this

Event trigger = entity dies --> execute command "worldborder add -5 20" in name of event/target entity

this seems to do nothing when entities of any type die ingame (I've tried using source entity and it didnt help. I'm also sure I've got the correct command)