The line comes from Warcraft III: Reign of Chaos, released by Blizzard in 2002, where Arthas Menethil is a human paladin and prince of Lordaeron. It is one of his attack acknowledgements when ordered into combat in the real time strategy game. As Arthas later became the Lich King in World of Warcraft, his early voice lines stayed widely quoted, shared on soundboards and in gaming videos as a concise, recognisable insult from a major Blizzard character.
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Arthas from Warcraft 3 says Foul Knave in this short attack voice line, commonly used in gaming videos, meme reactions, and fantasy edits for a dramatic insult.
This is a brief attack command from Arthas Menethil in Warcraft III, delivered in a stern, aristocratic tone as he calls an enemy a foul knave. The line lasts just over a second, with clear enunciation and a cold, commanding edge that makes the old fashioned insult feel both serious and slightly theatrical. It works as an instant character moment, giving any scene a sense of medieval authority, disdain and dark fantasy without needing any surrounding dialogue or explanation to understand the mood it creates.