From the Super Mario Wiki
Dolphin Slash is Marth's up special move in the Super Smash Bros. series since Super Smash Bros. Melee, and is also used by Lucina in Super Smash Bros. for Nintendo 3DS / Wii U. Marth or Lucina quickly leaps up into the air and slashing their sword directly up, damaging any nearby opponents. The attack deals the most damage at the beginning.
In Super Smash Bros. for Nintendo 3DS / Wii U, both Marth and Lucina have the same two custom variants for this move. Crescent Slash gives more horizontal distance at the cost of jump height, and deals the most damage at the midpoint of the attack. Dolphin Jump increases the move's jump height, but does not damage opponents.