Here are all 5 hidden Achievements:

Grim Dawn 5 hidden Achievements

Difficult Decisions
Resolve the situation in New Harbor without resorting to violence.

In the Depths, He Lived Forgotten
Slay Gollus, the Deepdweller (Royal Hive)

Den of the Lost | Guide and Location

No Exceptions
Slay Anasteria (The Outcast)

Anasteria (The Outcast) is a Possessed witch in Fort Ikon Prison.

The Bane of Cairn
Slay the Loghorrean, Voice of Ch?thon (Final Boss)

The Death Room
Survive the Chamber of Souls. (Steps of Torment)

Ivdeo (YT) link here

The Fields of Despair
Survive the Fields of Despair. (Bastion of Chaos)

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