Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
expugnātĭo, ōnis, f. [expugno], the taking of a place by assault, carrying by storm, storming: urbis, Caes. B. G. 7, 36, 1: oppidi, Suet. Claud. 21: Mytilenarum, id. Caes. 2; cf.: ut ipsorum adventus in urbes sociorum non multum ab hostili expugnatione differant, Cic. de Imp. Pomp. 5, 13: castrorum, Caes. B. G. 6, 41, 1.
In plur.: nocturnae aedium, Cic. Att. 11, 23, 3: nunc acie, nunc expugnationibus, Vell. 2, 98, 1.