The Phantasmagoric Inferno is a Biome created in the Nether. It mostly consists of Ash Blocks and Magma Blocks. This biome is one of the most useful Nether biomes added by Biomes O' Plenty, as the Ash can be crafted into coal or be used as fuel in a furnace. You can easily spot this biome by the huge amount of Ash Blocks. Before 1.11.2, Smoldering Grass Blocks were used instead of Magma Blocks.
As of early April, 2016, Nether Biomes have not be re-added to 1.9 and 1.8.9 BoP, though the programmers are planning to bring them back. They are in 1.7.10 and earlier, though.
However, in version 1.11.2+ all nether biomes except Boneyard are readded to BOP.