You might be surprised by how ambitious and detailed framedarchitecture's Civilisation V mod is. While other mods only change a few things, his attempts to fully convert the entire game to one of the largest and most famous campaign settings Dungeons & Dragons has to offer: the Forgotten Realms.
You can choose from 25 civilisations, all reflecting Faerûn's largest kingdoms and races.
Policy branches have been replaced with classic D&D schools of magic.
The tech tree has been fully revamped with new buildings and units that fit into Faerun. Gotta go for that Wild Magic upgrade!
And, of course, the religions too, have been replaced. You can finally worship the Cult of Cyric...
Oh, and Drizzt is in the mod too!
Faerun for Gods & Kings [Steam Workshop]