Sources:
Daggardon, Copper Dragon
Halzaran (himself), Beholder Prince
Quardox, Death Slaad
Lumiere, via Helm of the Wolf
HALZARAN: What you know or have surmised!
His primary lair is north of Tarne, is called Monsmör. Locals know to avoid the location for many reasons, but primarily because it is displeasing environment. Most have unsettling feel for the area and it seems as you approach, there seem to be ever-watching eyes. It lies nearly 5 miles north along the Selin River as it approaches the wilderness that lies past the Pass the bares its name.
Halzaran, as all powerful beholders tend to do, has subjected other beholders to his will. They no longer can fully exercise free-will communicate and tend to follow his commands through his advanced telepathic mind. With this loss of will, and desire to protect their master, sometimes these pathetic beholders tend to blaze into battle without using their superior intelligence. Halzaran has often released his captive companions and sent them to establish new hives. There are 3 hives outside the main hive of Monsmör.
Halzaran lives in his Hive Room above the main hall. It is outfitted with multiple traps and devices to destroy trespassers. You have seen it with your own eyes.
There is more that drives Halzaran. It seems he had reached a basic alliance with Isok the Lord Vampire of Angor as well as with a powerful Slaad network based to the east. None are sure of his real motivations since a Beholder Prince tends to despise all other powers as usurpers to his extreme power. However, one must understand that a Vampire Lord and a Monstrous Death Slaad are of a different variant to this basic principle.
Regarding his involvement with the Black Hand: He established the Black Hand in Loden and Ironthal. For some reason he has overlooked the town of Tarne mainly because it has nothing of value and is inconsequential. He has a teleportation circle in Monsmörto reach his partners in evil. Now with the fall of Angor just reaching Halzaran, he clearly sees opportunity and looks for new alliances that will solidify his power and expand upon his realm. It seems that Helios Verum is a group worth such an alliance. He hoped to appease Quardox with gifts of Black Obsidian for further influence in cities along the eastern sea. You have helped him achieve that influence and he is grateful.
That's epic.