Complex sentence: Arthur Stengos, who is now just a head with red flesh growing over him, explains to Natasha and Grigory that the brains of everybody in Tranquil Repose are being used to metamorphosise into new Dalek mutants.
Split into simpler sentences: Arthur Stengos, who is now just a head with red flesh growing over him, explains to Natasha and Grigory what's going on. He tells them that the brains of everybody in Tranquil Repose are being used to metamorphosise into new Dalek mutants.