ensis

Ensis, ensis is third declension masculine and an i-stem, so genitive plural ensium. The nominative in -is tempts people into treating it as feminine; it is not. Beyond that the declension is unremarkable.

What matters about ensis is register. It is the poets' sword. Prose writes gladius; Virgil, Ovid and Lucan write ensis, partly because ense and ensem scan where gladio and gladium do not. So finding ensis in a text tells you something about the writer's ambitions before it tells you anything about the weapon. Later Latin verse and hymnody keep the habit. It left almost nothing to the Romance languages, which took their swords from spatha instead - French epee, Spanish espada, Italian spada.