The altar was built by followers of Moloch
A bleeding red Ruby in the size of a human palm is floating 1m above the shrine.
The Ruby holds a portal to the Ethereal Plane close, if it will be moved or destroyed the portal will open.
The Cultists built this place to keep a Scroll of Summon Greater Demon scroll safe there, with Ethereal Plane monsters protecting it. The scroll was hidden inside the shrine and the Ruby is there to divert attention from the shrine itself.