Omnislash is the final Limit Break Cloud learns in Final Fantasy VII. The attack can be learned by using the item of the same name won in the Gold Saucer's Battle Square, provided Cloud has learned all of his other Limit Breaks. When used, Omnislash deals 15 hits against randomly-selected enemies, inflicting massive total damage. It is famously used as the finishing blow on the game's final boss, Sephiroth, regardless of whether or not it was learned by Cloud beforehand.
In most Final Fantasy games, magic spells and special attacks are prefaced by the appearance of a text box showing the move's name at the top of the screen, hence the "Omnislash" text box at the beginning of the Final Smash.
Also in most of its appearances, Omnislash often involves a frenzy of flight-based slashes on the foe (barring in Kingdom Hearts and Kingdom Hearts (Re): Chain of Memories where it only involves three or a few more slashes in slow succession), though its appearance and the amount of slashes performed varies based on title. Most of the time, a downward slamming strike is the recurring finisher in those cases.
In Smash Bros., the move's animation stays true to its appearances; after an initial slash to hit the opponent(s) into the air, the attack consists of 15 sword slashes (the same amount used in Final Fantasy VII), with the final hit being the downward slamming strike from Final Fantasy VII and Kingdom Hearts.
Upon activating it, a text box with the word "Omnislash" appears at the top of the screen, while Cloud prepares his Buster Sword in a flash of light and wind. He announces ツキがなかったな. ("Your luck runs out.") before charging forward (in a similar fashion to Critical Hit, Marth and Lucina's Final Smashes), stopping at the first opponent he tackles. Cloud cannot cancel the move once started, unlike Critical Hit, so missing with this attack on the ground on a walk-off stage/in the air without any intervening terrain usually results in a self-destruct. The tackled opponent (and any other targets immediately around them) are knocked high into the air with Cloud's Buster Sword. Caught fighters are repeatedly slashed in midair while the camera dramatically pans around Cloud and his victims, before Cloud charges up a final blow, slamming him and his opponents to the ground in a mighty explosion, taking a total of 50% damage. The move can potentially KO as early as 15%.
Opponents who are caught only by Cloud's concluding sword slash (not by the initial tackle) take anywhere from 10%/14% damage (depending on how many targets are ultimately hit by the explosion and their distance from Cloud).
Omnislash does not affect and is not affected by Limit Charge in any way.
Cloud sets out to prevent Sephiroth from sending Meteor crashing into the planet. The fate of the world on his shoulders, Cloud confronts his enemy one on one. He unleashes a flurry of swift slashes before delivering the finishing blow. In Smash Bros., the final strike creates an explosion, launching nearby fighters.