Here is the full list of all 18 Phantom Abyss achievements.
First death in a temple
First Relic
Purchase a blessing
Claim a relic in The Ruins
Claim a relic in The Caverns
Claim a relic in The Inferno
Bless a whip
Unlock all tier 1 whips
Unlock all tier 2 whips
Unlock all tier 3 whips
Bless all tier 1 whips
Bless all tier 2 whips
Bless all tier 3 whips
Own all relics from The Ruins
Own all relics from The Caverns
Own all relics from The Inferno
Reach Divine 3 rank in Adventure mode
Break 3 chains binding Altec in The Abyss