Algalon and its Heroic mode equivalent concludes the Algalon the Observer attunement quest chain, rewarding the . Doren will offer this quest once all four sigils have been turned in to him. The followup quest is the reward for key-bearers who defeat Algalon.
You really did it, <name>. Take the sigils to the Archivum Console and see if you can gain access to the Celestial Planetarium.
You best hurry, <lad/lass>. Time is of the essence.
Stand by for verification.
Access to the Celestial Planetarium granted. Please retrieve your key.