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