Berserker
CondDmg
Bonus damage while you are below 50% HP.
Rolls between 8% and 40%, scaling with the gear score of the item carrying it.
Forge mods that grant it
Spending one of these puts the perk on your gear instead of waiting for a roll.
Rolls on
BowSwordWandStaff
A drop or a craft of these types can roll this perk into one of its slots.