Mega Charizard X
Mega Charizard X has two attacks: the standard attack button executes Dragon Rush, corkscrewing in the direction the Circle Pad or Control Stick is tilted, and the special attack launches Fire Blast at opponents, trapping them in a multi-hitting flame. Dragon Rush deals 24% while Fire Blast deals 16% damage, and both can KO at around 110%.
Unlike Super Dragon, Charizard is not invincible, and taking damage will cause it to transform back to normal sooner. However, it takes only half of the normal damage and has infinite super armor, making it impossible to flinch.
If it hasn't taken any damage, Mega Charizard X's transformation will last up to 12 seconds.
In Smash 4
Mega Evolution is a mechanic introduced in Pokémon X and Y. Only a relatively small amount of fully evolved Pokémon can Mega Evolve, and to do so they must hold a special Mega Stone unique to their species. A single Pokémon on a trainer's team can Mega Evolve once during each battle (or in Contest Spectaculars as of Pokémon Omega Ruby and Alpha Sapphire). Mega Evolution dramatically boosts the Pokémon's stats (and sometimes decreases stats in favor of others) and changes the Pokémon's appearance, cry, and sometimes its ability or even its typing. Charizard can Mega Evolve by holding onto a Charizardite X or a Charizardite Y as its item. Charizard is one of two Pokémon (the other being Mewtwo) that have two Mega Evolution forms, the two forms being an "X" form and a "Y" form, with the particular Mega Stone only appearing in Pokémon X and Pokémon Y respectively. Both Mega Stones can be obtained in all of the games that follow (barring Pokémon Sword and Shield, where Mega Evolution is not present).
Mega Charizard X specifically induces the most drastic color change of all Mega Evolutions, turning Charizard from orange to black with blue flames. It also changes type from Fire/Flying to Fire/Dragon, and its Attack, Defense and Special Attack stats are increased. Additionally, its Ability changes from Blaze or Solar Power to Tough Claws, which increases the power of all direct-contact attacks.
Fire Blast (だいもんじ, Daimonji) is a Fire-type special attack with a base power of 110 (120 in Generations prior to VI) and a base accuracy of 85% that's parallel to a few other moves (along with a 30% chance to burn a target). Charizard's evolution line is one of the select Fire-types that can only learn this through TM38 in all generations, while others may learn it naturally through leveling up. Its Japanese name is a term for an "uppercase/big letter", hence the kanji for 大 (dai/ō; lit. "big" or "great") is what the attack takes the shape of.
Meanwhile, Dragon Rush is a Dragon-type physical attack that debuted in Generation IV, and is also used as one of the custom move variations for Flare Blitz in Smash. This makes Charizard one of the few Smash characters that includes a custom move variation in its base moveset and from a Final Smash.