When they find a human man, they envelop him with the black orb, making it flow into the man. After having his entire body violated by super high concentrations of demonic energy, the man is instantly transformed into a powerful incubus. By merging with the black orb, the man's desire and the desires of the black orb are combined, he will then be ruled by extreme lust. Once this happens, the man will end up giving in to desire, having sex with the remaining part of the dark matter, as a substitute for the black orb. After obtaining spirit energy in this way, they produce a high volume of magical power and scatter it all around.
Although it mentions "merging with the black orb", you don't turn into a shapeless blob of tentacles.