The Infernal Bulb is a component added by Roots. It has a 1/20 chance of being dropped from Nether Wart. It is used for various spells. It is also used as a solid fuel, burning for as long as a Blaze Rod. Additionally, right-clicking a Skeleton with consume the Internal Bulb to turn the Skeleton into a Wither Skeleton.
Runic Tablet entry
Infernal Bulbs will drop at a 1/20 chance from grown nether wart. Infernal bulbs are imbued with the fire of their home in the nether, so they are useful when a large amount of energy is needed. They can also be used as fairly effective furnace fuel, or given to skeletons to make them withered.