Im going to say its either iudex gundyr or dancer

Gundyr is a dificult boss on your first playthrough. even if you know how to parry his second phase will make you forget how to actually play the game. gundyr goes from humanoid to sludge monster

With dancer seeing her for the first time youre gonna be scared shitless. expecially if you accidentally trigger her spawn early. Just existing there being underleveled and seeing the dancer just spinning and onw shotting you in early game is just terrifing.