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