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Memory Beta, non-canon Star Trek Wiki

The Imperial Legion was a military branch within the Romulan Star Empire that Centurions belonged to.

Centurion Karon belonged to the Legion in 2400 in the alternate timeline created by the War of the Prophets. (DS9 - Millennium novel: The War of the Prophets)

Presumably, the Legion is the arm of the Romulan military that consists of its ground forces during wars.