Sleeper

The Sleeper was the last of the first-generation Nightlords, super-vampires created back in the second age. His "sister" was the being who became Lloitira. They were lovers, but also hated each other. The Nightlords were basically unkillable at the height of their power, but were vulnerable to being sealed away by mass sacrifice. Thousands of people were killed to lock the Sleeper (whose true name was Kender) away behind two sets of barriers.

First is a physical maze, littered with traps and with the entrance caved in. Those that died serve as undead sentinels--undead because the nightlords have power only over living minds. The final door forms a complete barrier/doorway to a demiplane; unless opened from the outside, no means of physical exit is possible.

The second are a complex set of arcane seals placed in multiple planes for two purposes. The first is to prevent mental contact or projection with/from the Sleeper. The second purpose is to drain away its energy to reinforce the maze, keeping it at low power (ie "asleep"). There are multiple sets of these.

After thousands of years, the Sleeper has substantially weakened. At full strength it would be of demigod status (CR 25+), capable of bending minds and growing a cult while feeding on the emotions and mental strength of those around it. Of course as a fully-demonic creature, it would also then get banished to the Abyss...unless it had a worship base to counteract the draw.

At the time of its destruction in 250 AC, the remaining defenses were:

• The inner maze (the entrance and outer maze have been mostly cleared by slaves)
• All of the draining seals and some of the barrier seals.

It made contact with the captain of the Scarlet Pearl via an amulet that Ahzekiel Cai'Valur discovered in a ruin (while enslaved), formerly belonging to a Sleeper cult. That focus and Ahzekiel were the primary contact points. However, even if the amulet is destroyed, enough of the seals have been broken for it to start influencing those around it more directly. The White Skull manipulated events and the contact so that the Sleeper was focused on removing the seals and believed that Enigma had come to finally free it from its prison, which let them get close enough to end its existence. The demon prince also betrayed its true name to the party, enabling its final and complete destruction and the release of the souls it had stolen over its existence.