This sound comes from Undertale, the 2015 indie RPG created by Toby Fox. The battle-start cue became familiar through gameplay videos, speedruns and fan edits because it instantly tells viewers that a fight or encounter is beginning. Online, creators use it as a shorthand for sudden conflict, surprise challenges and mock turn-based battles. Its staying power comes from how short and readable it is: even people who have not played Undertale often recognise the dramatic game-battle setup it signals.