Under the assumption that 'Neimodian' is the name of a merchant raider...(!) "The Neimodian had seized many freighters under the pretence of stopping pirate activity." or "The Neimodian had seized many freighters under the cover of stopping pirate activities." And mix/match cover/pretence (British English, apologies) and activity/activities. The 'cover' or 'pretence' are both valid when used this manner. With respect to 'pirate activity/activities', this is a weird quirk of English. The phrase uses the word 'pirate' as a noun adjunct, since the word 'pirate' isn't technically an adjective. 'Activity' is valid due to its intangible definition (a catch-all for everything the pirates are doing), and 'activities' is valid, too, due to the tangible definition (incorporating all the individual things the pirates are doing).

The Neimodian had seized many freighters under the cover/ pretense of stopping piracy.