Congealed Blood
Congealed Blood is a block created when a minecart full of blood goes down a hill at high speed. The blood flies out and forms a block. See Minecart Blood.
Placing congealed blood above or below Mob Spawners will cause them to always spawn mobs, even when no players are nearby.
Congealed blood can be used to summon a Demon using a Demon Altar.
Congealed blood smelts into Blood Ingot.