Nyeheheh

An interjection made popular by a magic skeleton named Papyrus in the game Undertale, made by Toby Fox

Papyrus: NYEHEHEH!

nyehehehe

A particular laugh made by Papyrus in the game Undertale.

Also, an 'evil' type cackle.

Sans: I bought more spaghetti
Papyrus: NYEHEHEHE YES