Has-Been Heroes

This achievement requires you to finish most of the story of Has-Been Heroes by completing a minimum of seven full runs of the game.

First finish six full runs for each of the elements. Then, each subsequent time you complete a run you can choose who opens a final chest. Select Tam (the starting 3-hitting Rogue) to open the final chest. This will unlock Thanatos as a playable character, as well as the achievement.