Hardcore death in <5 seconds but i don't know what happened?

The Druant there has two modifiers:
Spiteful and Rampancy. Spiteful causes it to gain bonus damage for four seconds when it's hit, stacking up to 10 times, and Rampancy causes the enemy to intermittently Rampage, granting temporarily increased size, damage, movement speed, and damage reduction.

It cast Entangling Roots on you, which connected rooting you in place, and dealing physical damage over time. When it cast this, it had both 10 stacks of Spiteful, and was actively Rampant causing it to do a lot more damage than normal. The leap slam was the final nail in the coffin. Certainly a deadly combo.