Marth (SSBU)/Side special
Dancing Blade can be used as a standard combo ender. It should be avoided in neutral as it is very unsafe on shield, with most opponents having enough time to dash forward and grab before Marth can jump. His safest option if a non-ending hit does hit a shield is to continue using Dancing Blade, delaying hits in hope that the opponent drops their shield. It is Marth's third fastest out-of-shield option, tied with neutral aerial. It is slightly easier to perform than up aerial (second fastest by 1 frame) and doesn't leave him as vulnerable as whiffing his fastest out-of-shield, up special.
The down-angle ender provides the most damage. The side-angle ender sweetspot can KO very early on, being able to KO several characters below 70% at ledge (30-50% earlier than the non-sweetspot hits). The side-angle ender's sweetspot can be hard to hit as it has lower priority than the weaker hits, requiring the attack to be delayed for the highest chance of hitting the sweetspot. Because of various other factors such as rage and directional influence, hitting the sweetspot is inconsistent and it is best to plan around a non-sweetspot hitting instead. Forward tilt is a good, grounded alternative if the side-angle ender's sweetspot is too difficult to land.
The first hit can be used to attack an opponent recovering along the stage, making it a safer alternative to up special (which has the potential to face the wrong way and likely lead to an SD). It also serves as a mixup option, as the tech timing is different than that of an up special along with requiring different reaction timing, potentially resulting in the opponent buffering an air dodge if they've been conditioned to expect up specials. The first hit can also be used to reorient Marth to properly use an aerial or correct his direction to more safely perform an up special.
While recovering, the first hit can be used to slightly halt his downward movement. This can make his recovery slightly less predictable, along with being much safer against opponents that have high-commitment edgeguards or to intercept opponents who go for an offstage KO.