Galaxia Darkness is Meta Knight's Final Smash in Super Smash Bros. Brawl. Meta Knight throws his cape over the unlucky player and the whole screen goes dark. Then, you see large slashes and all of the players goes flying, usually to their doom. If Meta Knight misses someone while using Galaxia Darkness, nothing will happen.
This final smash is relatively easy to perform. Simply swing the attack in front of the player and any player caught by the attack get a large knockback attack dealing about 40%. The remaining players get hit with a smaller attack dealing 10% and small knockback.. In Brawl, the move also hits the players right behind Meta Knight.
Changes in later iterations
Super Smash Bros. for 3DS / Wii U
There are almost no changes to the move. The hitbox for both the cape swing and attack so, only those completely touched by the cape swing get affected by the full blast of the attack. Standing behind him is also relatively safe unless they are right behind. It has the same damage output but it can kill at 30% or higher.
Meta Knight's Final Smash. He flips his cape, and the darkness within envelops his foes and plunges the world into shadow. He then unleashes a series of sword strikes that do terrible damage and launch his foes. In the darkness, Meta Knight can also reach and attack distant enemies. One can imagine Meta Knight zipping back and forth at high speed to accomplish this.