My first night in Giza House! One of the maids had lighted a fire in the bedroom and the flickering flames threw their shadows over the walls. How different from those of the Dartmoor cottage; these seemed like sinister shapes which would assume life at any moment. How silent the house was! There was a door behind a blue velvet curtain. I opened this and saw that it led into the room where the sarcophagus had been.
If you "assume a position", you put yourself in that position.
If you "assume command", you take control.
If something "assumes human form" it changes into something that looks human.
In this excerpt, the shapes look as if they could come to life at any time.
See: assume (verb)