Stephen King's The Mist Wiki has a more comprehensive article on the same creature. For it, see http://stephenkingsthemist.wikia.com/wiki/Tentacles"Tentacles" is the nickname for an extradimensional being that attacks the Federal Foods Supermarket in The Mist.
Numerous squid-like tentacles with black spines on them kill Norm in the storage room. The suction cups on the Tentacles serve as mouths, consuming Norm as the tentacles envelop him. Ollie gets a fire axe and attempts to cut Norm loose, but the tentacles pull him outside before he can do so. David Drayton manages to kill two of the tentacles with the axe and the beast retreats, allowing Ollie to start up the generator and shut the door. The severed tentacles quickly decompose into an acidic, black liquid.
The story never directly explains what the tentacles are attached to, although several characters engage in light speculation on the matter. They are from another dimension (possibly Todash space).