Warhammer: Chaosbane

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

• Quest: The Whispering Wood
• Boss: Bloodthirster

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 host a solo game and load their last save, which should put them less than 5 minutes from the boss battle (thanks TheOnlyMatto for the heads up!).

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 Bloodthirster 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.