Comet is a magic spell in Final Fantasy VII Rebirth. It is granted by the 
Comet Materia, and is the lower-level comet spell below 
Cometeor. It is a powerful non-elemental magic damaging spell that fires a number of meteorites in an area around the targeted enemy, dealing immense damage overall. However, the spell is hindered by its high MP cost and very lengthy cast time.
Use[]
Comet can be used when the comet materia is equipped. The spell deals immense damage in total, spread across multiple meteorites striking over an area, giving it strong area-of-effect damage against multiple enemies or powerful damage to a single target. However, its MP cost and lengthy cast time mean it needs to be cast carefully.
Comet's non-elemental damage means it is effective against all enemy types regardless of elemental weakness. It also means that, against enemies with an elemental weakness, it is still more efficient to target them with a spell against their weakness. Additionally, in cases where the player wants to hit a single target with a non-elemental spell, Quaga may be a better way due to its shorter cast time and greater reliability.
The Comet spell is at its best when its shortcomings are alleviated by 
Support Materia. One option is Synergy Materia, which allows a party member to cast it with frequency and with no MP or ATB cost, though it still leaves them vulnerable to being hit during the cast time. Another great pairing is Swiftcast Materia, which can mostly eliminate the lengthy cast time and make Comet much less risky to cast.
Aside from these choices, a few other support materia can be used with Comet. HP Absorption Materia, MP Absorption Materia, and Magic Focus Materia can have their uses. Magnify Materia is effectively redundant as the spell already has a great area-of-effect, and Auto-Cast Materia is very risky to use.
Using Comet with Aerith's Arcane Ward will cast it twice. Further, using Yuffie with Doppelganger when in Arcane Ward can cast Quake four times, potentially overwhelming enemies. If this is done, it is best to use swiftcast.
Etymology[]
A comet is an icy small Solar System body (SSSB) that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet.