The Phantasmagoric Inferno is a Biome created in the Nether. It mostly consists of Ash 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. Blue Fire now appears in this biome, along with regular fire. Blue Fire gives the curse effect when stepped into.
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.
Blue Fire was added to this biome as of Biomes O' Plenty version 1.12.2-220.127.116.111.
As of 1.12.2-18.104.22.1680, magma blocks no longer generate in this biome.