Here is the full list of all 50 Eldest Souls achievements.
Defeat Eksyll in Abyss Mode.
Defeat Aryana in Abyss Mode.
Defeat The Warden of Purity in Abyss Mode.
Defeat The Deer God in Abyss Mode.
Gain the Blessed Trueice Prism
Defeat Drakmur in Abyss Mode.
Defeat Hyem in Abyss Mode.
Defeat Azikel in Abyss Mode.
Defeat Eos in Abyss Mode.
Defeat the Guardian in Abyss Mode.
Defeat Eksyll
Reach the Steps of Ascension
Gain the Abyssal Eye
Defeat Drakmur, the Nightmare
Obtain the Empty Vessel
Maintain 4 stacks of Berserker for 30s.
Deliver the bag of crowns to Edelon.
Defeat Aryana, First of the Moon.
Defeat Hyem of the Frigid Waste.
Gain the Blessed Feather of Dawn
Defeat The Deer God and the Warden of Purity.
Reach Concord Sanctuary
Defeat Azikel, God of Light.
Hit enemies with a Shard attack 200 times
Defeat Eos, God of Unity
Hit enemies with a thrust attack 200 times
Retry a Boss 10 times in a row.
Hit an enemy with Windslide, Berserker Slash, or Counter 20 times.
Reach the Crossroads
Defeat the Guardian.
Die to a Boss.
Talk with Rass
Deliver the harp string to Edd
Defeat the Watchdog.
Die to a rolling log.
Reach Crusader's Landing
Defeat the Watchdog while taking 0 damage.
Defeat the Guardian while taking 0 damage.
Defeat Azikel, God of Light while taking 0 damage.
Defeat Eos, God of Unity while taking 0 damage.
Defeat Hyem while taking 0 damage.
Defeat The Deer God and the Warden of Purity while taking 0 damage.
Defeat Aryana, First of the Moon while taking 0 damage.
Defeat Drakmur, the Nightmare while taking 0 damage
Perform 3 Perfect Counters in a row against any Boss.
Have at least 10 Windleaves active at once.
Reach the ending without dying once.
Finish the game without activating any Shard or Infusions.
Gain the Kindled Heart of Fire
Gain the Blessed Hunter's Charm