Church of Myrkul

Faerûn god of the dead, also called Lord of Bones, Lord of the Dead, and The Reaper.

Once powerful necromancer named Myrkul Bey al-Kursi from Murghôm (far to the east - ruled by Dragon Princes - like actual dragons!)
The necromancer ascended to godhood alongside hero-villains Bhaal(violence and murder) and Bane (tyrrany, terror and hate)
The Uthtower, sunken beneath the Mere of Dead Men is said to rise from the swamp when a star appears in the sky called the Eye of Myrkul, bringing with it a growth in power of its priests and legions of undead from the sunken kingdom.

(Important additional information in: Mere of Dead Men and Uthtower)

Led by a powerful and ancient necromancer / high priest called Ularan Mortus.
Ularan is said to have become an undead creature as part of the sacrifices in the creation of the dracolich Ebondeath.
Ularan has emerged, even after thought to have been destroyed, several times over the centuries in the attacks emerging from the Mere and the Uthtower.
The dread forces have included a gigantic warship created from the dead and bound with the soul of a powerful Death Knight named Emberlost.
After destroying much of the town of Leilon in 1490 DR, undead minions have continued to harass all in the region - including an escalating rivalry against the Cult of Talos the storm god, who has recently taken from Ularan the Emberlost in a sea battle near the Tower of Storms.