Charms have Weapon or/and Armor Bonuses.
A charm with a Weapon Bonus means that the bonus effect occurs when you attack an enemy.
When the charm has Armor Bonus, it means that the effect occurs when you get hit by an enemy.
Therefore, an epic charm that has a chance to produce a Gravity field that kills enemies outright with Armor AND Weapon bonus means that there is a chance to create said "Blackhole" when you attack and get attacked.
Currently, I'm using one with armor bonus and I see the gravity field appearing alot.