Here is the full list of all 14 Skelethrone: The Prey achievements.
You died at first
Defeat The Boss - Stray Guardian
Defeat The Boss - Mardek`s Machine
Defeat The Boss - Champion Sacrass
Kill all hound's enemies without losing your life
Сollect all Shards in a Level
Find a Pilgrim and talk to him
Check out the promotional menu
Defeat a hundred common enemies
Defeat The Boss - Stray Guardian without damage
Defeat The Boss - Mardek`s Machine without damage
Defeat The Boss - Champion Sacrass without damage
Complete the Prologue in less than 30 minutes
Get All The Achievements