Bolvagon is a powerful and vile beholder, currently imprisoned in the Abyss. The heroes first encountered Bolvagon when the creature successfully launched an attack on the city of Amollo, using a dominated portalmaster and his army of dark dwarves. Petrin Iceblade was captured during the attack and was taken to the Abyss where Bolvagon planned to use him as a means to gain freedom.
To be free of the Abyss the beholder needed to be invited out. Bolvagon attempted to talk Petrin into making the invitation, but the dragonborn had no intentions of doing such a thing. The beholder even gave Petrin a gift, an imp familiar named Narullian. Petrin accepted the gift, but when his companions opened a portal to rescue him, instead of inviting the beholder through, he made a run for it and escaped.
Since his escape, Petrin wondered why a creature that could read minds let him go…and give him a gift no less. It made him uneasy. Later the companions would find out that Narullian is a vessel of Bolvagon’s essence (referred to as a Remnant of Bolvagon). By taking him from the Abyss, the beholder’s influence in the world became much stronger.
A later conversation with Kimath It-Arist revealed that Bolvagon was behind the creation of the Book of Years. To save his parents, and perhaps Petrin and Kitten, Eldarion needed to destroy that book. To do so, he would first have to weaken Bolvagon, which meant destroying the four remnants of Bolvagon that existed in the world.