Demon Altar

A demon altar is a multiblock structure which is used to summon Demons.

A demon altar consists of a block of lava, 8 blood blocks surrounding the lava on all sides (including diagonals), torches on the 4 corner blood blocks, and any solid block underneath the lava.

When a flying block of Congealed Blood lands in the lava, it is destroyed and a Demon appears on one of the corner blocks. The blood must land in the lava, not just pass through it, which is why there must be a solid block underneath the lava.

If you already collected the Congealed Blood, a Demon Item Mover might still be able to help you trigger the altar.