Complex sentence: Dalek Sec opens up his casing and uses his long tentacles to grab Diagoras, enveloping him in his own body before dragging him into his casing to begin the'evolution'.
Split into simpler sentences: Dalek Sec opens up his casing and uses his long tentacles to grab Diagoras. He pulls him closer and envelops him in his own body before dragging him into his casing to begin the'evolution'.