Warhammer: Chaosbane

This achievement is story-related and can't be missed.

• Quest: City of Blood
• Boss: Great Unclean One

Achievement should pop after beating the boss and leaving the area.

WARNING: It might not unlock for Guests (Players 2, 3 & 4) if playing co-op.

Edit: If that happens and you managed to immediately quit the game, the guest should try hosting a solo game and load their last save, which should put them a few minutes from the boss battle.

If the glitch occurred but you and your partner advanced past the boss battle, try these workarounds for the account who has it missing:

• Host a game and beat Great Unclean One on Boss Rush mode (didn't work for me);
• Create a new character, host a new game and speedrun on Very Easy difficulty;
• Create a new character, host a new game and bring your co-op buddy as guest so they can carry you using their main (higher level) character.

Personally, I used the third option to unlock the achievement for my wife after we had completed the game. She started a Level 1 Elessa and I joined her game using a Level 50+ (Legendary 85) Elontir. I simply speedran every quest and she got the achievement after the boss fight.