Living Fire Block

Once you are in Hardmode, any enemy killed in the Underworld can drop a pile of these, the Wall of Flesh included, and they come in stacks rather than singles, so an ordinary trip down usually leaves you with more than you need. Statue spawned enemies never drop them. If you want a lot in one go, summon King Slime down there, since every slime it splits into rolls the drop on its own.

Placed, it lives in the background layer. You walk straight through it, it does no damage on contact, and nothing collides with it. It links to other fire blocks both sideways and vertically, but ordinary blocks will not connect to it, nothing except platforms can sit on top of one, and torches refuse to attach. Block Swap does not work on them and a hammer does nothing, although sloped versions exist and can be placed with an outside world editor. Compare Wooden Beams, which only stack vertically but do accept other blocks on them.

Light is the main reason to keep a stack. They are free, easy to restock and they go places a Torch cannot, including under water and inside lava, since liquids do not put them out or wash them away. The limit is housing: they count as neither a light source nor a background wall, so a room lit only with fire blocks will never pass as a valid home.

The plain block survives being dropped into lava, the same way Obsidian and fireblossoms do. The recolored ones do not, so carry those carefully if you are decorating the Underworld with them. All the tints are made from the base block and behave the same otherwise. The Living Demon Fire Block ties with Shroomite Plating for the longest crafting chain of any block, and the Living Ichor Block is the one member of the family whose name leaves out the word fire.