Byleth (SSBU)/Neutral aerial
Spins Failnaught around himself, hitting multiple times. Byleth's fastest aerial attack. It is useful for edgeguarding and approaching, has some combo potential, and it also possesses a landing hitbox, making it a useful multi-purpose aerial attack. Byleth may occasionally say "Failnaught!" while using this move. At its highest, it does 10.5% damage overall.
The Nairquake Loop is an advanced combo that involves the use of repeated neutral aerials, with a fast fall during each one to get the landing hit. While it usually won't kill or confirm into kill moves on its own, when done properly, it can deal heavy damage, set up edgeguarding scenarios, and lead into other combos and kill confirms.
The exact number of repetitions in each loop varies depending on the character. A list of how many repetitions can be done on each character can be found here.