Mega Evolution

Mega Evolution is a Final Smash of Charizard, Lucario, and Mewtwo. It is based on the Mega Evolution system from Pokémon X and Y, followed by Pokémon Omega Ruby and Alpha Sapphire, which enhances a Pokémon's power to a great margin. In most cases, whenever a Pokémon holds a Mega Stone, it becomes capable of Mega Evolution; the number of Pokémon that are capable of Mega Evolution, however, is limited, with one of them only being able to Mega Evolve if it learns a specific move.

In the Super Smash Bros. series, Mega Evolution appears as Final Smashes for Lucario, Charizard, and Mewtwo. When either Charizard, Lucario, or Mewtwo gains the power of a Smash Ball, they gain Mega Evolved forms that greatly enhance their powers, and makes them impervious to flinching. They can still take damage, however.

Mega Charizard X
Mega Charizard X gains the ability to fly, which is similar to Yoshi's Super Dragon. It has two attacks: Fire Blast, which allows it to shoot fireballs that spread into a kanji 大 upon contact, and Dragon Rush, which it rams into the opponents in a similar way to Flare Blitz move.

Mega Lucario
Mega Lucario's aura-based attacks will always strike at maximum power, regardless of its damage. The transformation only lasts for a short time period, however, and its duration gets shortened even further if Mega Lucario takes any damage. Unlike most Final Smash transformations, Mega Lucario maintains all the custom specials that its normal state has.

Mega Mewtwo Y
Mega Mewtwo Y will attack opponents with Psystrike, dealing damage to them. It is currently unknown if that's the only action that it will perform though.