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