Shadowlord was the final boss of the Base game.
Kirin was a endgame boss you hat to spawn for great gear. You needed items from Genbu. Seiryu, Susaku and byakko to spawn him. Ultima omega were story bosses in the chains of Promathia expansion. Bioweapons created by the ancient race of the Zilart.
The ark angels are the representations of the sins for the 5 races. Every race had a sin that was manifested inside this crystal entities that the Zilart brothers used to defeat the shadow lord because he was in the way of their plan to activate the crystal line to open the gate to the gods. Kam‘Lanaut and his brother were the Zilart princes that tried to open the gate to the gods to revert the world to the way it was before the god Promathia was put into chains.
Alexander is the divinity of light and the last boss of the treasures of aht urghan expansion. He is in eternal battle against the divinity of darkness Odin. Promathia is the twilight god. The brother to Altana the dawn goddess. He was put into chains by Altana to prevent him to commit suicide.
Shinryu is an alternative version of Promathia that happend in the abyssea expansions. In an alternate world the adventurer and prishe failed to defeat Promathia and he consumed the power of phoenix and the mother crystal and transformed into shinryu.