Miltiades's Divine magic reveals a strong aura, evil natured flowing from the black altar. The specific school is undeterminable because there are too much schools of magic in this altar.
Searching the four sided column where Korbryn just pointed, Thandis finds the narrow slit and the secret button, inside a secret compartment you find a skin-covered black drum.
Blessed Glittergold, it's a human skin, blasphemy!
The gnome calls
Searching around the column, Thandis find that every one of the remaining three sides have secret compartments as well, in the first one you find a small bead. Miltiades finds the bead to be magical (Moderate evocation).
In the third side, the secret compartment holds a set of silver chimes. In the fourth side's compartment you find a bronze brazier and three chunks of incense.
Could be ceremony tools … but to whom? The elder elemental eye?
The Historian gnome goes back and forth, trying to remember something