Here is the full list of all 20 Nightside achievements.
Complete the Prologue mission
Complete Story Chapter I
Complete Story Chapter II
Complete Story Chapter III
Complete the Story Mode
Complete a tutorial mission
Beat the AI on Easy Difficulty
Beat the AI on Medium Difficulty
Beat the AI on Hard Difficulty
Win a 1v1 Multiplayer game
Win a 2v2 Multiplayer game
Win 10 Multiplayer games
Win 50 Multiplayer games
Win 100 Multiplayer games
Win 10 games vs AI as Nova
Win 10 games vs AI as YX
Win 10 games vs AI as Human
Win 20 MP games as Nova
Win 20 MP games as YX
Win 20 MP games as Human