As with "Azn" Krn is a retarded, shortened form of the word Korean. Those professing to be Krn are but fuckheads, claiming pride in their hardcore ability to rush opponents in starcraft and being general arsehole nuisances with their breaking voices and tryhard auras.
Person 1: That guy, Song, he thinks hes soo hard?
Person 2: Shit man! He's KRN!