Journey through Madness
Cyric (pronounced SEER-ick), is, without being too biased, a complete and utter scumbag. We are talking about a deity here who has a reputation for betraying anyone and anything for personal gain. As a deity, he is universally reviled outside his church following, and stands for murder, conflicts, lies, betrayal, and illusions. He has a history for taking great pleasure in tricking mortals into making decisions that completely ruin their lives. As you have likely guessed already, he is a chaotic evil deity.
His church supports the same concepts as he, and this means that (surprise!), his church is also universally reviled. This is mainly because his followers murder and try to instigate wars and civil unrest, which are generally unpopular with the populace of civilized locations.
As a mortal, Cyric fought beside Midnight (see: Mystra) and Kelemvor to prevent the destruction of the magical balance. During the height of all this, he betrayed his comrades, and killed Kelemvor. As Kelemvor and Midnight (see: Mystra) were at this time lovers, Mystra has made it a personal point (and rare departure from her otherwise benevolent nature) to cause Cyric suffering wherever possible.
After Cyric murdered Mystra, he was punished with permanent insanity by the other deities. This is a massive blow to Cyric, as his strongest asset was the complex schemes his mind devised. A group of deities sympathetic to Mystra took the further step of banishing Cyric for a thousand years.
His symbol is a skull super-imposed over a black sun.