SKAVEN. Given their technological bent these creatures deserve to be a power in the 40K universe.
Species targeted diseases from Clan Pestilens, genetically enginered war machines from Clan Moulder and god knows what from Clan Skyre. ( The thought of what they'd do when they get access to the Warp doesn't bear thinking about).
Lead by the immortal genius of the Council of 12 and with the personal aid of their actual god ( not some cannbialistic, corpse being), they have hid in the holes of the Warp for ages planning their final move aginst the galaxy, now the Warp rat-holes will open and the hordes will pour forth to bring the age of the Skaven top be.