Volcanic Deserts are underground deserts. Full of lava. So. Much. Lava. They also happen to feature mobs, but their main feature is lava. Have you heard that they have lava?
Being a desert and completely devastated by lava, Volcanic Deserts only support-if you could even call it that-Scorched Trees, which cannot burn and provide a black wood. They support no other flora. Obviously. (Actually, I'm kidding; they also support Prickly Pear Cacti).
Of course, no Erebus biome would be complete without a host of wild and dangerous mobs, including the dreaded Solifuge: