where am i getting this "unbreakable" buff from?

[G-Fresh]

it gives 10 hp regen/suppression for 10 seconds

[Burusagi]

If you survive being under fire for long enough you get.

Like the Kill Streak/Massacre bonus for killing enough enemies in short enough timespan.

[Ryvucz]

Angel of Death is a similar one. They trigger after a specific event/limit of not dying.

[G-Fresh]

ahhh ok ty