Each attack you perform against an enemy increases the damage it takes from your attacks by 0.80%. You can level this Legendary Gem all the way up to rank 150. Every upgrade will increase the damage gain by 0.01% up to a maximum of 2.30% per stack. The stacks are additive with themselves and multiplicative with all other damage bonuses. The secondary effect increases damage against Rift Guardians and Act Bosses by 25% multiplicatively.
First enemy hit with a Skill that can proc Bane of the Stricken receives a stack of debuff that increases all damage you do to that particular enemy.
After applying a debuff to an enemy, Bane of the Stricken goes on Internal Cooldown equal to “0.9/Skill APS”, where “Skill” is the Skill that was used to apply the debuff.
The above formula is only true for all non-channeling skills and Whirlwind, all other channeling skills make Bane of the Stricken go on longer cooldown than their attack rate would imply.
Generally only skills with a proc coefficient can proc Bane of the Stricken, meaning that Pets and item procs will give you no stacks. However, this is not a hard rule and there are exceptions.
Normal enemies and Elites lose all Bane of the Stricken stacks when they leave combat. Rift Guardians never lose stacks even if you as the Player die.
For a deeper look into Stricken mechanics and RGKs, read this guide from Chewingnom.
Bane of the Stricken Best Use
The only way to kill a Rift Guardian at higher Greater Rifts: Nearly every Solo Push build and every Rift Guardian Killer uses this gem to kill the Rift Guardian in an adequate amount of time.
Legendary Gem in Diablo 3
Legendary Gem Creation:
If you need extra Legendary Gems to level up, you can make the Rift Guardian in any Greater Rift drop a new Rank 0 Gem by removing all the Legendary Gems with the same name from your Equipment, Inventory and Stash.
The safest way to do that is to put them into the Inventory of different character.