Kelemvor is the god of death. He's not exactly the "big kahuna of undead" unless you redefine "kahuna" as "nemesis".
For gods of undead, you have Velsharoon (he's rather the god of necromantic magic), Kiaransalee (but she's only popular with the drow) and Orcus. Bane and Cyric likes necromancy too, but the undead aren't their core portfolio. It's rather just a hobby for the rainy weekends.
Myrkul is kinda dead, kinda still here. As far as I know, he trapped part of his essence in a cursed crown, and possess a few people with it, from time to time. He doesn't seem interested in reclaiming divinity back, but he still have a few cultists practicing necromancy.