First obtain both Chaos Avenger and Rank 10 Chaos Militia. Then open the Chaos Avenger Class shop at /championdrakath and purchase the regular Chaos Avenger's Greatsword. Upgrade it to the Empowered version in the Merge shop with 5 Champion Drakath Insignias. Find Champion Drakath strategies here.
Alternatively, you can get a non-stable weapon with the same boost, which is called the Empowered Blade of Chaos, which still requires 5 Insignias but no Chaos Avenger. Simply obtain the Blade of Chaos by defeating the Champion of Chaos at /ultradrakath, or if you have the Prime Chaos Orb cape (Rare), purchase it from /curio.