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