Curious Expedition 2

To unlock this you must have an Abomination party member, and another party member for them to eat.

The most reliable way to obtain an Abomination is to get sanity to 0, manual save, and then save scum Insanity Events until you get one where a party member gets a strange bite mark. I've also seen a random event when travelling in fog where a party member transforms into an Abomination instantly but this seems to be rare and I only encountered it once.

Wait long enough and the party member will turn into an abomination. This can take a while so make sure you have food or a mission/campsite nearby to rest at straight after the event as you'll need to keep the party alive.

Once they transform into an abomination, continue playing and after a short time you will trigger an event where they will try to eat your other party member. Choose to Look Away, the achievement will unlock and you will lose the other party member.

Obviously this is a pretty good way to end your run so recommend doing this separately to any completion achievements.