Dark Army

Old-school Nyx's forces, as led by one very nasty man named Guylos. No, not that Guylos.

The uncreatively named Dark Army had a lot of very powerful Zoids, many of which snacked on Deochalcum to be even more powerful. Deochalcum also gave them their distinct green glowing underbits; the black/charcoal/glowing green "Dark Zoid" scheme is one of the more recognizable and popular in fandom. Even Takara-Tomy acknowledged as much, with a recent series of limited recolors to go with Rebirth Century.

Both Tomy and Shogakukan first introduce the Dark Army and their Zoids by having them wipe out the remains of Zenebas' army—including a Death Saurer, which in the Shogakukan version is raised into the air and crushed by a Deadborder's Deochalcum-fueled Gravity Cannons. They then go on to destroy the Republic forces that investigate. One take on this includes a Deadborder with a spear mercilessly tearing apart all manner of Zoids.

In other words, the Dark Army does not mess around. In fact, they're flat out ruthless, and are often described as not stopping until their opponents are completely destroyed. About their only weaknesses are bright light/heat (being from Nyx, which is cold and often overcast) and their lack of air Zoids, something which they later made up for with the introduction of Gilvader and Gungy.

By post-meteor Zi, the Guylos army is portrayed in a much more sympathetic light, more like the Zenebas army was in the original line. Granted, a lot of their troops now are former Zenebas soldiers Guylos absorbed, but the change seems to have been a deliberate attempt to get away from the simplistic "good versus evil" feeling that plagued the late portions of the original line.