"The altar of hecatomb...The skull of the deceased..."
The Hecatomb Altar is an end-game furniture block, and is one of the essential things needed to summon the last Hardmode boss, The Doom. Aside from being possible decoration, it serves an actual purpose and it is to serve as a part of the process to summon The Doom. Unlike other 'tables', things can only be placed on the right side of the Hecatomb Altar.
If a Cryptic Old Skull is placed on the Hecatomb Altar, it will become yellow. It can then be rightclicked to trigger the following two messages:
"Fleshilite has appeared in the world!"
"You have offered a great future sacrifice. Be prepared."
Upon these two messages, Fleshilite Ore will appear throughout the world's underground. It is about as rare as Orichalcum Ore. Mining Fleshilite is the next step to being able to finally summon The Doom.