Here is the full list of all 10 Song of the Deep achievements.
Destroy the Forbidden City forever
Commune with the giant seahorse
Defeat the Fomori Boss
Defeat the Watcher
Activate the Deeplight
Meet the Merrow
Feed the Queen Leviathan
Purchase all upgrades
Kill a Tremorclaw Crab as Merryn
Pay your respects to the Watcher and drop a flower on her final resting place