Nethengeic Withers are the abomination that occur when a Wither is infused with the essence of a Nethengeic Beast.
They are an extremely rare occurrence, probably due to the rarity of the Wither itself combined with the isolated nature of the beasts.
Not much more is known about them due to both their rarity, and their intensely aggressive demeanor, but it is generally accepted that they are not naturally occurring creatures, but are purely constructions of fused essence.

Regardless, much like either of its counterparts; it is extremely dangerous and any time spent in its proximity is time spent peering into the finality of death.
Run, while you can.