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