Aolar

The demon lord Aolar was killed, and her Abyssal fortress destroyed, by a vengeful Desna in 4155 AR after the Lady of the Hunt took control of the body of a recently deceased Desnan priestess to wreak death on the priestess' loved ones. This possession of heroes just after their deaths was Aolar's penchant, especially when the dead person's soul could witness the suffering caused.
Aolar made many enemies in this way including Calistria, Sarenrae, and Shelyn, who all helped Desna in her vengeance. Calistria's aid especially was key, as Desna's flagrant act nearly started a planar war: Calistria's politicking turned the allied demonic host, the Coalition of Chaos, against itself and averted war.
Like all dead demon lords, Aolar's quasi-soul now resides in the Abyssal Rift of Repose.