A couple things come to mind:
Blade of the Tiere - A battered, pitted iron knife that contained the dying hatred of a god who was sacrificed with the blade. It didn't quite have an ego, but it kept nudging the holder in the direction of anything so much as tainted by the presence or lingering presence of the person who had killed that god, or the thing it had been sacrificed to.
The Word of Rescindment - one of the Words, of which the Last Word of Orcus/Tenebrous fame is most known. This one was effectively a do-over button for anything the holder did, or had witnessed, removing spell effects and even entire sequences of events. The price however was rather high for its use, scaling with what it was used for.
The Opener of Ways - another one of the Words, this one being a universal key of sorts. Doors, wards, portals, etc.
Staff of the Lower Planes - the artifact staff once held by Anthraxus, lost following his death in my campaign. Located at the behest of the Overlord of Carceri and returned to her by the PCs in an evil one-shot game I ran.
The Shadow Sorceled Key - cursed artifact that acts as a universal portal key to any portal reaching the lower planes, including gates within Sigil. Created by the baernaloth Lazarius Ibn Shartalan, and had a tendency to fall into the hands of those naive enough to unwillingly abuse it and cause their own ruin.
A complete set of the Nether Scrolls - obtained by the PCs for a fiend IMC. They didn't know they were giving him the set till they opened the box and he took them. Oddly enough this didn't come to haunt them.
Heavy Magic - liquid magic. Touch it and think of an effect and you might just get it, virtually without restriction. But it's unstable, hellishly so, and even when trying for a minor effect like a replicated spell, there's a percentile chance of the user being incinerated, crippled, stat drained, some unexpected result happening, etc.