The Shaper is the creator of the Atlas of Worlds and the main antagonist of the Atlas of Worlds expansion. He acts as a final boss, residing in The Shaper's Realm. The Shaper also occasionally appears in maps that are influenced by him.
The Elder is a boss that is invading the Atlas of Worlds. He is the main antagonist of the War for the Atlas expansion. According to the lore of the Conquerors of the Atlas expansion, the Elder was slain by the Elderslayers led by Sirus, Awakener of Worlds, and was removed from the Atlas.
Sirus was the leader of the Elderslayers, and according to the game lore, disappeared along with The Elder after defeating him, much to the dismay of Zana, who grew attached to him during their time together. Once a hero, Sirus turned mad after defeating the Elder and then destroyed Oriath, forcing the player to eventually fight and defeat him.
The Maven is an NPC and boss encountered within maps. Progressing the Maven's questline allows the player to unlock the Maven's Beacon, which lets the player call upon the Maven to the map to witness you. Defeating enough unique bosses while the Maven is witnessing you will earn you a Maven's Invitation to challenge them all at once for rewards.