It’s applied as it’s own damage type.
You can’t really increase it in any way.
Most bosses have massive innate resistance to it, rendering it worthless.
It’s based on total health.
It’s a damage type that for the most part, only works vs players, who for the most part either have 0% or even -25% resistance to it, with only a grand total of 3 items giving resistance to it. Only 2 enemies in the game use it consistently, on an easy to dodge projectile skill. Every other instance of it is on weapons for a very small value.