"Thanergy is the energy of death. Handling it always has repercussions for a mortal being's health," she says tersely.

"You will know when I have overextended myself. My nose is not even bleeding yet."

They are making good time back towards the stairs, despite moving backwards. The scuttling remains in front of them, and Harrow has the uncomfortable feeling of being herded.

Her bootheel strikes the last stair. At the same time, a curiously light footfall lands on a step above, and then another. Something is coming down to meet them.