The Temple of Molag Bal is a room that is upgraded by Castle Volkihar Redux. This area already existed in the Volkihar Ruins. Once the Dawnguard quest Touching the Sky is completed the ruins become Valerica's Tower and it will upgrade this room with new shrines, organs, decorations and NPC's.
The temple was originally just a broken down Temple of Mara in the old Volkihar Ruins. When upgraded the temple becomes a lavish fully functioning temple again. This time it's for Molag Bal.
Many new items are added to the temple. Two playable organs, custom decorations, shrine, altar, large statues depicting worshippers, and a lectern on podium catwalk. A small office is located somewhat hidden behind the catwalk near the Alter of Molag Bal (both the altar and the shrine are interactable). There are many church pews to sit on, two minions and a high priest of Molag Bal.
A Castle Guard also stands at attention on the bridge overlooking the temple.
An obscure storage chest can be found next to a desk on the upper level of the catwalk. It contains some random high level treasures including 5,000 gold and a full set of custom Robes of Molag Bal.