Here is the full list of all 50 Eldest Souls achievements.
Defeat Azikel, God of Light while taking 0 damage.
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.
Gain the Blessed Trueice Prism
Defeat the Guardian while taking 0 damage.
Defeat Hyem in Abyss Mode.
Defeat Azikel in Abyss Mode.
Defeat Eos in Abyss Mode.
Defeat the Guardian in Abyss Mode.
Perform 3 Perfect Counters in a row against any Boss.
Defeat Eksyll
Hit enemies with a Shard attack 200 times
Reach the Steps of Ascension
Gain the Kindled Heart of Fire
Obtain the Empty Vessel
Defeat Drakmur, the Nightmare
Gain the Blessed Feather of Dawn
Defeat The Deer God and the Warden of Purity.
Defeat Aryana, First of the Moon.
Defeat Hyem of the Frigid Waste.
Deliver the bag of crowns to Edelon.
Reach Concord Sanctuary
Defeat Eos, God of Unity
Hit enemies with a thrust attack 200 times
Retry a Boss 10 times in a row.
Defeat Azikel, God of Light.
Have at least 10 Windleaves active at once.
Reach the Crossroads
Defeat the Guardian.
Hit an enemy with Windslide, Berserker Slash, or Counter 20 times.
Deliver the harp string to Edd
Die to a Boss.
Talk with Rass
Defeat the Watchdog.
Die to a rolling log.
Reach Crusader's Landing
Defeat the Watchdog 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 Blessed Hunter's Charm
Gain the Abyssal Eye