After questing in Zangarmarsh for awhile, someone finally has the idea, "Hey, all of this mysterious, horrible stuff starting happening to the swamp about the same time that the mysterious giant pumping station popped up in the middle of the lake," so they send you down there to check it out. You swim underneath the pump, and resurface in an underground cave that leads to four instance portals. These are the dungeons and raid that make up Coilfang Reservoir. The easiest dungeon is the Slave Pens.
This is where the naga are holding all of the enslaved Draenei who are powering the Reservoir's operation. The first boss is nicknamed The Betrayer, so we can infer that he played a part in handing over his friends into slavery to the Naga.
The next boss is a large lobster-like creature with thick armor and a powerful attack.
The final boss is an enormous fungal giant who is just kind of hanging out underwater, not really hurting anyone. Naturally, you need to massacre him.
The Underbog is fairly uninhabited by Naga, since it appears to be under the control of the marsh creatures themselves. The first boss is a fungal giant named Hungarfen who throws paralyzing mushrooms at you.
A bit later, you fight a gigantic hydra that the Naga appear to worship...
and another Draenei traitor, a hunter who is living on his own deep in the cave.
The final boss is a giant fen strider named the Black Stalker, a nasty plant that shoots electricity.
The Steamvaults is the hardest of the Coilfang Reservoir five-mans, meant for Level 70 players. This is the heart of the Coilfang Reservoir operation, and the place where the power for the massive pump is being generated.
The first boss is a Naga mage, with two water elementals to assist her.
Up next is a deranged leper gnome, who has sold his services to the highest bidder.
Finally, comes a Naga warlord, who is in charge of the Steamvault operation.
Coilfang Reservoir also houses a Level 70 raid, Serpentshrine Cavern, where players have the opportunity to take it to one of Illidan's closest allies, the Naga queen, Lady Vash'j.