Necrotic/Black fire

It's a black fire can be made with wither dust dropped by wither skeletons and Withered wraiths.

This can be used to make necrotic fire variants of all the fire light sources that stop non-wither mobs from spawning in a 5 block area around them regardless of light level.

Necrotic fire gives wither effect on top of fire damage. By left clicking the ground with wither dust you set a necrotic fire (kind of like a fire charge). Wither dust can be crafted in to a way to locate the nether fortress.