Similar to Roy's Flare Blade in function, Ike lifts his sword, Ragnell, and then plunges it downward in front of him, resulting in an explosive burst of flames. It can be charged for up to four seconds, causing the fire to greatly increase its range and knockback. The knockback caused by this move is mostly vertical when not charged fully and becomes more horizontal when it is. Like most of Ike's moves, it is powerful enough that charging is not required for it to be effective (in Brawl stage 1 can KO as low as 106%). The flames reach high up, so there is a large vertical hitbox as well. There are also smaller hitboxes below, far in front of, and behind Ike. In Super Smash Bros. Ultimate, Eruption now produces up to two additional pillars of flames in front of Ike depending on its charge, increasing its range even further. This attack, like Aether, possesses armor, with it applying from the moment that the special button is released to the point where the sword hits the ground. As Ike is particularly susceptible to juggling due to his high weight and falling speed, the armor frames can be useful for defending against it.
A fully charged Eruption (or stage 7 to 9 in Brawl) can break any character's shield, provided that it was not a perfect shield. It can also be a meteor smash when sweetspotted on the tip at the very end of the hitbox, but except for Brawl (where it can one-hit KO regardless of the stage) the spike is weak and only works on grounded opponents, making it impractical and rarely used (if at all) in competitive play.
In Brawl, most neutral special moves can be reversed by tapping the control stick in the opposite direction a few frames after the neutral special move has been activated. This includes Ike's Eruption; however, reversing it has no animation and no increase to damage or knockback, making it not very useful unless used to attack behind Ike in midair. It is interesting to note that when in Ike's running animation and activating a reverse Eruption, Ike will turn around and slide forward a little while charging or releasing the attack; this is probably because the running momentum has switched direction with Ike, since there is no animation for the reverse Eruption and has pushed him forward as if he didn't turn around at all.
Eruption has 9 stages of power: 8 charging-up stages and the fully-charged stage. A fully-charged Eruption has a larger hitbox and a horizontal trajectory while dealing 10% recoil damage back to Ike, like Roy's Flare Blade. Ike will start flashing red and glowing once he reaches stage 7.
A stage 8 Eruption is Ike's most powerful attack, strong enough in Brawl to KO at 0% against lightweights. However, charging the move to precisely that stage is very difficult, as the player must release the special button exactly on the frame just before the attack becomes fully charged. A stage 8 Eruption takes 228 frames to charge; if released a frame early or late, the move will be charged to only stage 7 or will be automatically released as fully charged, respectively. Stage 7 Eruption could potentially one-hit KO lighter characters (specifically characters not heavier than Pikachu), but usually starts to KO Mario at 8% on Final Destination. If Eruption gets copied by Kirby, it is slightly powered up, as usual for most copied moves. Now stage 7 can KO Mario as low as 5% and can OHKO characters as heavy as Zelda.
Eruption is incidentally the only OHKO attack among playable characters in Brawl that neither is a Final Smash nor relies on specific conditions (like Thunder), countering or counter scaling (such as Oil Panic). Eruption loses its OHKO power in Smash 4 and Ultimate, with a transition to the fully charged version about 3/4 of the way into charging, preventing the strongest stages from being reached.
The following is a video demonstration of the nine stages of Eruption in Brawl.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
In the Fire Emblem games, neither Ike nor Ragnell have fire-related powers, making Eruption a creation unique to Super Smash Bros. The closest thing to an exception is the climax of Fire Emblem: Radiant Dawn, in which Ike wields the powers of the goddess Yune for a brief period of time to defeat the final boss; Yune's power manifests as blue flames when channeled by Ike, hence Ike's Japanese epithet "Hero of Blue Flames" (translated as "Radiant Hero" in English versions). While this does not match the red flame seen in Brawl, the flame effects are recolored blue in SSB4, presumably to better match the source material.
Visually, Eruption's explosion bears a strong resemblance to the rare and powerful fire spell Bolganone's effects, which summon a volcanic eruption underneath the feet of a foe; Ike is not a mage and cannot use Bolganone, although some of his allies can after they have been sufficiently leveled up.