This item, aside from being a port of Michael Moorcock's famous black, soul-sucking blade of sorcery, is intended as a signature item, so it gets stronger as the owner advances through the levels. This and other magic items I've created are collected together in the attached PDF.
Stormbringer: For each enemy slain by Stormbringer, the wielder gains 1d8 hit points (2d8 at champion tier, 4d8 at epic.) For each battle the sword is drawn and it does not slay at least one enemy, the wielder loses 1 recovery until the next full rest (2 at epic tier,) to a maximum of half their total recoveries.
Quirk: Dark Addiction. After the first time a character uses Stormbringer, they become unable to relinquish possession of it willingly; only death can break the magical addiction. In addition, the sword has an unsettling effect on any who see it drawn, whether they have knowledge of such things or not.
History & Lore: More so than other magic items, Stormbringer has a personality, and a voice. Malevolent and insidious, it requires constant sacrifice in order for the wielder to have what passes for peace (“peace” being a somewhat relative term when one finds themselves in close proximity to the Black Blade.) A weak minded being would be wholly unable to unsheathe the sword, the physical discomfort would be too great. But even one able to overcome that barrier would risk their mind being broken by the will of the sword.
In Stormbringer's presence, most all beings, animal and man alike, are at a minimum unsettled. The effect becomes noticeably more pronounced when the sword is drawn; even creatures with no magical knowledge or sense can feel the naked thirst for life it has. As if to reinforce its status in its relationship with its wielder, it will rarely allow itself to be sheathed without taking a life, and when it does, it's at a steep cost to the one holding it.