Rogue Assassination Artifact Ability Useable By All Specs (Patreon)

I am receiving reports that the rogue assassination artifact weapons ability "Kingsbane", is useable by all specs using different artifact weapons, when the quest to obtain the assassination artifact weapon is run in a different spec.

The Kingsbane ability is normally activated by "The Kingslayers", the assination artifact weapon. From the tooltip: " Releases lethal poison within The Kingslayers  and injects it into your target, dealing (240% + 240%) Nature damage  instantly and an additional (252% of Attack power) Nature damage over 14  sec.
Each time you apply a Lethal Poison to a target affected by Kingsbane, Kingsbane damage increases by 15%. "

The Kingsbane spell appears in the rogue's spellbook in other specs.

Obviously, this is a very overpowered ability when used in conjunction with another artifact weapon.