Nagas are hatched from defective dragon eggs. They embody the arcane power and venomous potential of dragons.
The Elysombra PCs, at 12th level, met six spirit nagas guarding the Church of Hell in the gnome city of Vindle. The nagas, with their repeated fireballs, killed one character and handed the rest of the party their first loss in battle.
To run monsters quickly in combat, I prepare combat sheets, summarizing their abilities. You can download my spirit naga combat sheet (.rtf file) to make running nagas in your own campaign faster and easier.
(I've managed to get sheets like this officially posted to the Wizards web site, too, for use with a previous miniatures line.)
The pages of the combat sheet include:
Magic Treasure: Each naga has a magic item as its treasure, with the item's effect statted in.
Prepped and Unprepped: One sheet shows you what a spirit naga is like if it hasn't put up any defensives spells. The other sheet shows the naga with 7 spells up, for a naga that's had time to prepare for battle.
Combat Checklist: Hit points and spell slots for lots of nagas, so you can keep track of several of them at the same time.
Nonstandard Details: This is for my campaign, so you'll see some details that aren't standard. You can work around them.