Thorium is a radioactive metal (atomic number 90, natural form Thorium-232) used in a synthesized liquid form as the poison for The Norseman device. The 'standard' form of thorium has radon-220 gas, which is poisonous, as one of its decay products. This was the toxin used to poison Sarah during her rehearsal dinner by Vivian Volkoff, who assumed it would kill her almost instantly.
Presumably the Norseman takes its name at least in part from the god of thunder (Thor) element involved.
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