Here is the full list of all 24 Legacy of Dorn: Herald of Oblivion achievements.
Find the Lightning Claw
Find the Plasma Gun
Find Brother Ordyss
Find Brother Barathon
Find Brother Kael
Find all of Squad Scipio
Find the Chainsword
Find Librarian Radamus
Find the Thunder Hammer
Get inside an Ork Stompa
Find the Blessed Blade
Destroy the Herald of Oblivion
Destroy the Herald of Oblivion and live
Escape the Herald of Oblivion
Find a Signum
Sabotage the plasma reactors
Sabotage the ancient warp-field generators
Find the Master-crafted Storm Bolter
Find Sergeant Valerius
Find the Relic Blade
Find the Power Sword
Find the Heavy Flamer
My enemy is my friend!
Trick a dreadnought to its doom