ItemEmpowered Chaos Avenger's Greatsword.
Complete the 13LoC story then do the storyline from /shadowwar > /dualplane, and /forestreach > /deepforest > /transformation > /downbelow.
Member + Non-Mem
Complete Iaste's "Expect the Inquisitors" quest from /crownsreach by defeating Inquisitor Guards at /citadel. Requires Rank 10 Chaos to see this quest.