Formication is the sensation you get when you enter a caesure, or Fall. Or it would be if you were a fictional character in the world of the Land. A caesure (not a real word, but it’s clearly modelled on ‘caesura’) is a kind of hurricane created by wild magic that sucks up living things and deposits them in another time and damages or destroys anything else it passes over.

for·mi·ca·tion /ˌfɔrmɪˈkeɪʃən/ [fawr-mi-key-shuhn]

–noun a tactile hallucination involving the belief that something is crawling on the body or under the skin. 

[Origin: 1700–10; < L (Pliny) formīcātiōn-, s. of formīcātiō a sensation that ants are crawling on one’s skin, equiv. to formicā(re) to have such a sensation (v. deriv. of formīca ant) + –tiōn– -tion]