To get Stars of Ruin in Elden Ring , you need to progress Sorceress Sellen’s questline far enough to receive the Sellian Sealbreaker , then find Primeval Sorcerer Lusat in Sellia Hideaway and speak to him. The spell is tied to that hidden dungeon in Caelid/Dragonbarrow and is obtained by opening Lusat’s seal, not by picking it up from the ground.

Steps

  1. Get Comet Azur first from Primeval Sorcerer Azur in Mt. Gelmir.
  2. Speak to Sellen in Waypoint Ruins to continue her quest and get the Sellian Sealbreaker.
  3. Go to Sellia Hideaway in northeastern Caelid, near the Church of the Plague.
  4. Find the hidden entrance behind the graveyard headstone / illusory wall.
  5. Work through the cave until you reach Lusat’s sealed room.
  6. Use the Sellian Sealbreaker to open the barrier and talk to Lusat to receive Stars of Ruin.

Fast route

A common shortcut is: Church of the Plague → northeast graveyard → break the hidden entrance → follow the crystal path down → open Lusat’s seal. That is the most direct path once you already have the Sealbreaker.

Important note

You cannot get Stars of Ruin without advancing Sellen’s quest enough to obtain the Sealbreaker first. If you go to Sellia Hideaway too early, Lusat’s barrier will still block you.

TL;DR: Get Comet Azur, talk to Sellen, obtain the Sellian Sealbreaker, then open Lusat’s prison in Sellia Hideaway and speak to him for Stars of Ruin.