Here is the full list of all 17 Nith Realms achievements.
Obtain a x20 combo in any combat
Perform a finisher on any enemy
Obtain a x15 combo in any combat
Pet the dog "Bruno" in the fallen realm
Pet the dog "Kala" in the mountain realm
Pet the dog "Lio" in the fire realm
Pet the dog "Milka" in the ice realm
Pet the dog "Milu" in the ice Realm
Pet the dog "Wiki" in the agony realm
Defeat "Sardious" in the fallen realm
Defeat "Cathro" in the fire realm
Defeat "Fentor" in the ice realm
Defeat "Hunsa" in the agony realm
Defeat "Azhel"
Play any minigame in the mountain realm
Obtain a x30 combo in any combat
Perform a ground finisher on any enemy