The mask on its own means nothing to him - it's unfamiliar and Raiden assumes it's just your generic supervillain fare - but the voice. No specific words come to mind, but there's hazy memories of pain and terror and that voice rattling around his memories. Smug and poisonous.

"Not long enough," he backs up a step, almost a giveaway as to how totally freaked out by this whole thing he is, but he puts a hand on his sword to counteract it, "I'd suggest that you leave."