cut the 20 charges to 10. this is reasonable, as for example gamasters and dreadneughts could just spam those abilies without actualy fighting, and even a fury of the wild could easyly get that 10 vorpal strike, while this ability requires actualy killing something.
holy damage wouldnt rly fit a darker themed destiny, but definetly need to increase the unholy dmg part. oh and make it so that i dont have to reequip my handwrap, as going in and out from "epic moment" loses some of my weapons abilities (shockburst for example)
add the 30% chance to do the 10d6 unholy burst to mele atacks as well
to further buff this ability, maybe you could add energy drain, like 1 lvl/hit, on every hit or sneak attack (tod set grants some chance for energy drain, so this ability would fit perfectly). this would be still behind the gamasters superinstakill, and wouldnt do anything on rednameds/deathwarded trash/constructs and undeads, so stil not owerpowerd