Aether, known in Japan as Heavenly Air (天空 Tenkū), is Ike's up special move in Super Smash Bros. Brawl. He first throws his sword in the air and crouches, preparing to jump. During this time, he gains launch resistance. After the sword hits the apex of its trajectory, he jumps up, catches it, and hurtles toward the ground. The move gives very little horizontal recovery, and as a result people have called it among the worst recoveries in the game. However, others argue that the move has decent vertical distance and the fact that he throws the sword in midair allows him to avoid edge-guarding, allowing the move to be a decent recovery.
If the attack is initiated in close range, it hits the opponent upwards and sucks them into a multi-hit combo in midair, which finishes with a spike when Ike swings the sword down and descends with the final hit canceling the spike. As a result, this move can also be used as an effective edgeguard. If Ike stands on the edge facing away from the platform and perform Aether, then instead of falling off the stage to his death, he turns around and grabs the edge, sending the opponent downwards quickly, often to their death, as few characters have a good enough recovery. However, due to being unable to grab the edge until descending it can be easily edge-hogged. Aether is also a good follow-up to his down throw, and its disjointed hitbox can be used to hit enemies through solid platforms, interrupt enemies' aerial attacks, or set off Quark Mines in the Subspace Emissary. It can inflict between 1% to 23% worth damage.
An interesting note is that if Ike uses Aether six times without landing on a platform (i.e. by grabbing an edge after each Aether), he will not be able to grab an edge on the sixth use, and he will simply fall to his death if there are no platforms beneath him. This is to prevent Ike from stalling on an edge, and is important to know when using any technique involving Aether for Cruel Brawl.
 Description from the Instruction Booklet
Throw the sword high into the air, jump to catch it, and then descend in a two-handed strike.
Aether is a powerful skill available to Ike upon promotion in Fire Emblem: Path of Radiance and Radiant Dawn, and later to Chrom and Lucina (or any of Chrom's other female children) in Awakening. When unleashed, the skill deals two strikes to an enemy, combining the effects of the skills Sol and Luna. In Ike's use of the skill, it involves throwing his sword upward, jumping, catching it, and coming down with a spinning cut that heals Ike equal to the damage inflicted (the effect of Sol), followed by a second rising strike that is performed as Ike puts space between himself and his opponent, which halves/ignores the foe's Defense score (the effect of Luna in Path of Radiance and Radiant Dawn respectively). However, neither effect is applied to Aether in Brawl, and the second strike is omitted completely.
If Ike is hit while throwing Ragnell in the air and is still on the ground when under the effect of the Timer, he might perform his landing animation for Aether while Ragnell is still in midair, creating 2 Ragnells. He will be stuck like this until Ragnell lands on the ground. Ike will then pick up the Ragnell he held and the duplicate will disappear. Note that sometimes, Ike will simply just stop or Ragnell will instantly land without spinning.
Ike uses Aether in The Subspace Emissary to prevent the Ancient Minister from dropping a Subspace Bomb. Ike shouts "Great Aether" as he attacks, although he is clearly not using said Final Smash (because "Aether" is said in two parts in japanese (天空 Ten-kū, lit. "Sky"), it was needed to fill the line).