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