Failnaught is Byleth's Neutral Special Move. Byleth uses his bow called Failnaught to fire an arrow, with its damage and knockback dependant on how long the player charges this move. If Kirby inhales Byleth and copies his ability, he can use this move as well.
This move, when used with no charge, launches an arrow with moderate distance, deals fair damage, and deals fair knockback. This move is not easily spammable due to the fact that it has a decent amount of startup. However, the startup lag can be canceled with a shield, which can provide Byleth with numerous amounts of baits and mixups. If the control stick is tilted in the opposite direction that Byleth is facing during startup, Byleth will turn around and fire the bow in the opposite direction, similar to Captain Falcon's Falcon Punch, Ganondorf's Warlock Punch, and Kirby's version of Failnaught and both of the mentioned powerful punching moves.
If the player decides to charge the move by holding down the special button, streaks of light will shine from Byleth's before an arrow that looks like a beam of light is released. The arrow will have a massive range, and if it manages to hit, the opponents caught in it will receive extremely high damage and knockback. However, unlike its uncharged version, Failnaught's charged version has significantly more startup and cannot be canceled with a shield. This means that using the charged version will make Byleth susceptible to attacks, grabs, shields, and reflectors.