My group killed Cyric in Forgotten Realms... by accident.
There was a nefarious plot, blah blah. All we were supposed to do was help Tyr by bringing him a broken shard from Godsbane and getting it forged into a dagger. In the midst of delivering it, Cyric had made his move and there was an epic god battle. It was supposed to be "in the background" as we avoided demonic soldiers and got the dagger to Tyr's throne.
The rogue, who had the dagger, thought it would be best to help Tyr and threw the dagger at Cyric. There were several celestials around, meaning Cyric was technically flanked. The ass rolled a 20 and scored the hit. Though it wasn't a crit, Godsbane had some kind of special properties against deities. I don't recall it all... but that attack turned the tide and won the battle for Tyr.
I didn't particularly care for god-killing (I think it's cheesy), but it's something I'd put on my resume'!