The phrase is a Portuguese League of Legends callout meaning there is a Vayne here, and the clip is associated with the name FakeMito. Vayne has a long reputation as a terrifying late game marksman who can melt tanks and duel almost anyone, so spotting her changes a match instantly. Brazilian players shared the panicked shout as a reaction sound, and it spread through gameplay edits and soundboards as shorthand for danger arriving. It stays popular because the fear is specific to Vayne but the reaction works for any sudden threat.