invader comes from invade (from Latin invadere 'to go into, attack', from in- 'in' plus vadere 'to go') plus the agentive suffix -er. Morphologically, the ending is part of the etymology: the suffix is attached to the older base named in the history, producing a later English derivative. In present English, the noun sense is "one who enters forcibly or intrudes, especially with hostile intent", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as invade, invasion, and invasive. Its semantic neighborhood includes attacker, raider, aggressor, and intruder, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Latin invadere and Latin vadere.