Orcusword

The Orcusword  was the weapon of Orcus when he was a balor. It was a +6 chaotic consumptive burst unholy greatsword that granted divine spellcasters (via Orcus himself) access to the Chaos and Undeath domains. It was kept on the Negative Energy plane in the Tchian-Sumere citadel, guarded by Alzogra the Bloated and Yleris, a yugoloth necromancer.

In the 2e campaign module "Dead Gods" the Orcusword takes the form of a Longsword +3, with the ability to deal double damage on a roll of a natural 20. Inversely, on a roll of natural 1, the sword would break.