Blackfathom Deeps is located amongst the ruins of a temple of Elune that stand along the northern end of the Zoram Strand in Ashenvale.
The entrance to the instance zone is reached by diving into a pool of water, and swimming up into the tunnel complex.
The tunnels and temple is primarily filled with Naga Sea Witches and Myrmidon's, as well as some Murloc's. Deeper in, you will be fighting stealthing Satyrs, and eventually a large regiment of Twillight Cultists lead by Lord Kelris.