Cyric

Cyric, whose titles included The Lord of Three Crowns, The Mad God, and The Prince of Lies, is the deity of strife and lies.

Ascension
Cyric, after slaying the god Bane, was granted his entire portfolio.

As a God
Cyric's activities over the years included things like tearing apart families, burning down churches and causing civil wars to break out in countries, as he subtly planted the seeds of deceit. He was known for taking absolute control and turning small favors that were owed to him into much bigger ones.

It all came to a head when he killed Mystra, instigating the Spellplague. The presumed motivation for that varies wildly, depending on the source, ranging from interest in the domain of magic, desire for revenge or even as a result of a misguided attempt at persuasion.

It is unclear why he would have wanted revenge in the first place, but it's obvious that Cyric blamed his former party for something, Kelemvor and Midfire more so than Adon.