ZeedMillenniumon is considered by many to be the strongest of all Digimon, and he's definitely either the 2nd or 3rd, surpassed only by Shoutmon X7 Superior Mode and Gaiamon. Considering he's a combination of so many Digimon, it's no wonder he's so immensely powerful. The process of creating him includes:
Kabuterimon + MetalGreymon (Virus) + Greymon + Angemon + Airdramon + Kuwagamon + SkullGreymon + Garurumon + Monochromon + Devimon (Black Gears infused) = Kimeramon
Kimeramon + Machinedramon = Millenniummon
Millenniummon + Devitamamon + GranDracmon = MoonMillenniummon
MoonMillenniummon + Laylamon + Axemon + SkullKnightmon + Blastmon + Gigadrmon + UltimateKhaosmon + Argomon (Mega) + Gulfmon + Duskmon + Megidramon = ZeedMillenniummon
Removing his bands = ZeedMillenniummon unbound.
The bands around ZeedMillenniummon are actually chains that suppress his abilities and power, and if they're ever broken he will destroy all worlds.
This can actually be broken down even more. For example, MetalTyrannomon + MetalMamemon + Andromon + Megadramon = Machinedramon, and BanchoLeomon + Darkdramon + Varodurumon + Kentaurosmon = UltimateKhaosmon, etc, but I focused on ZeedMillenniummon's main line.