Here is the full list of all 12 The Depths of Tolagal achievements.
Defeat the Hydra
Make it through the summoning rooms
Kill 2 enemies in the same turn
Make it through the deep moot
Defeat the Ashdurbuk
Make it through the buurzlug
Defeat the Maugoth
Make it through the old garrison
Make it through the forest
Get killed by a spike trap
Kill 3 enemies in the same turn
Kill 5 enemies in the same turn