Under normal circumstances Absol does not evolve from or into another Pokémon. However, in Pokémon X/Y a new form of evolution called Mega Evolution is introduced. With a Mega Ring (XY) or Mega Bracelet (ΩRαS), a high friendship, and the appropriate Mega Stone in hand a Pokémon can evolve once more, thus gaining in power, access to new Abilities and even changes to their types, giving Pokémon battles a whole new twist. However, unlike normal evolution it is not permanent and the Pokémon will revert back to its normal form once the battle is over.
Mega Evolution is not without its limits as well as it can only be used once per battle. This means if a Pokémon that has already Mega Evolved faints or you switch it out for another Pokémon that can Mega Evolve too the icon will be grayed out, meaning you cannot use Mega Evolution again until the next time you battle. Probably a good thing considering the amazingness of it all. Mega Evolution is also completely optional so if you decide not to take advantage of it that's totally cool too. I, however, like to use it as much as possible, just because I love it.
Another thing, too, is that Mega Evolution is limited to a small number of Pokémon. Thankfully, Absol is one of those special few that can use Mega Evolution. With an Absolite as its held item Absol can Mega Evolve into Mega Absol during battle. In Pokémon X/Y you can receive one from your rival in Kiloude City after becoming the Champion, and in Pokémon Omega Ruby/Alpha Sapphire you can find one somewhere in the Safari Zone. Although both ways of getting an Absolite are super late into the game, the wait is definitely worth it. Mega Absol is totally badass.
Aside from looking totally awesome Absol gains a boost to its Attack, Special Attack and Speed stats making it a lethal fighter. I like to teach it moves like Night Slash and Psycho Cut because of their high crit chances, and also Swords Dance to add to its already crazy high Attack Power. In addition, Absol also gains the Ability Magic Bounce which can reflect the effects of status changing moves or ailments back at the opponent making it even tougher to take down. However, Magic Bounce can also play to your disadvantage as I've recently learned while trying to capture a wild Latios where its Heal Pulse was reflected back to it and healed the Pokémon rather than me. Oops!