One of the most infamous magical weapons in Dungeons & Dragons, Blackrazor is one of the three artifact-tier magical weapons that first appeared in the AD&D module White Plume Mountain, alongside Wave and Whelm.
Blackrazor is essentially a rip off of Stormbringer, the soul-eating demon-blade wielded by Elric of Melnibone in Moorcock's dark fantasy Sword & Sorcery novel series. As such, Blackrazor takes the form of a greatsword with a jet-black blade, sometimes described as being filled with star-like glimmers of light. A physical embodiment of nothingness, sometimes stated as having been forged from a dying universe or something along those lines, Blackrazor is a soul-eating blade driven by an incessant hunger for the lives of others, and which compels its bearer to hunt and slay people. This traditionally gives it incredible lethality in battle, and makes it a handy way to circumvent certain unkillable enemies, but it also traditionally has a drawback - namely, it tends to react poorly to attempts to use it on constructs or the undead, usually resulting in the bearer taking damage instead of the target, as its unsated hunger causes it to gnaw at them instead.
Blackrazor reappeared in 3rd edition as a Legacy Weapon, a kind of "evolving" magical weapon. It then reappeared as an artifact weapon in 4th edition. Finally, in 5th edition, it was mentioned as one example "patron" for the Hexblade patron for warlocks.