The Cloud of Darkness

The Cloud of Darkness is a force connected to the Void: A force of nothingness. Whenever the balance of Light and Dark becomes skewed towards one or the other, the Cloud of Darkness manifests to return both the World of Light and the World of Dark to the Void. Previously it appeared thousands of years ago due to the Flood of Light, only to be halted by the Warriors of the Dark. Now, Xande unleashes it upon the world due to his messing about with the forces of darkness. The Warriors of the Light try to fight back, but the Cloud of Darkness is invulnerable and decides to begin by snuffing out their light. It is only by Doga and Unei reviving them and traveling to the World of Darkness to enlist the help of the Warriors of the Dark do the Warriors of the Light manage to overcome the Cloud of Darkness.

The Cloud of Darkness is an odd villain in the series: Most of Final Fantasy III centers around fighting Xande and stopping him from flooding the world with darkness. Sure, the Warriors of Darkness fought to stop the Flood of Light in the past, but there's no indication of said flood being sentient until the Cloud of Darkness appears. This trend would be repeated in a few places; Most infamously, Necron (or the Darkness of Eternity) from Final Fantasy IX is a similar force that appears out of nowhere to be a thematic final boss to its game. That said, Necron is just one extra fight rather than "THE IMBALANCE BETWEEN LIGHT AND DARK IS TOO POWERFUL TO LET THIS GAME END!"

One of the more obscured aspects of the Cloud of Darkness is in its alignment: Despite being called the Cloud of Darkness it actually relates more closely to nothingness. This is due to a number of reasons: It retreats to the World of Darkness (which it calls its own land in the Pixel Remaster) and it calls out the Warriors of the Dark for being willing to sacrifice themselves for the Light (despite having fought them before and trying to return their world to nothingness as well). However, while not directly said in the original, the Warriors of Darkness fought against the Cloud of Darkness previously during the Wrath of Light. While no material has directly shown how different, if at all, the CoD looked, the Dissidia subseries gave it a new design as an alternate costume known as the "Lucent Robe" which gives it more of a focus on light, creating speculation that it took a similar form (relatively speaking, as it likely took on the form of another giant naked lady).

It's worth nothing that the original Famicom version of III had a few minor touches to the Cloud of Darkness that were not carried over to later versions. For its overworld appearance, the Cloud took on a silhouette appearance; This was changed to a green energy orb in the remake and green fire in the Pixel Remaster. Similarly, the Famicom version had multiple sprites for different states: Its initial fight in the Crystal Tower (orange with blue and green tendrils), its first use of Particle Beam (Final colors with green and blue/red tendrils) and the final sprite. In the remake and the Pixel Remaster, all three battles use the same coloration.

Much like Xande, the Cloud of Darkness was not changed directly, but rather a number of changes around it impacted it. The most significant change is that the Cloud of Darkness now functions more directly as the true villain of the game. As the Warriors of Darkness explain; The Cloud of Darkness' appearance was as an unforeseen consequence of Xande attempting to freeze time by disrupting the balance. The Cloud of Darkness manipulates Xande to further its goals and, once time has frozen on the surface, is the one to send monsters to the Floating Continent to further its agenda. It is also the source of the Great Earthquake that starts the game, whereas the Famicom version had Xande do it.

Lasers

The Cloud of Darkness' go-to strategy in the original game is to spam Particle Beam as much as it can. It's pretty much a heal check; You need your healers to be in top form and your HP to be quite high to weather the beams. Occasionally, the CoD will take a break from the beams for a physical attack, which is bad because it can pierce defenses.

The remake makes some significant changes. The biggest is that now the Cloud of Darkness is accompanied by two tentacles; One focuses on buffing the CoD while the latter fires lightning attacks. It's up to the player to decide whether it's better to try to rush down the CoD or make things more managable by taking out the tentacles, but it's not that easy: One of the tentacles is immune to physical attacks while the other is immune to magic. On the other hand, the Cloud of Darkness has more moves and its physical attack no longer ignores defense. Interestingly, The Cloud of Darkness will refrain from using the Particle Beam until it loses 20% of its health.