An entity of extreme power, Leviathan is summoned to possess the combined bodies of Octo, Nebulous, Nami, Thermal and Aegon.
When Gills first read the code of Omens corpse, vestiges of the code contained lines that made no sense to King Gills. Placing these aside for further study he went on to rebuild Omen and the Air Warriors without the fragmented code section.
After the construction of Moray, King Gills had a breakthrough, completing the fragmented code with some of his own lines he placed this reworked code into the mudwarriors and Octo.
This code allowed for the merging of their steel bodies, their own personal codes are placed in statis and a powerful malevolent entity is temporarily downloaded into the newly formed body.
Even the size and strength of the combined body struggles to contain the powerful entity downloaded into it and damage to the body can cause the code to drop out and the body to separate.
This is lucky because combined, these entities are almost godlike forces to deal with, reaping city wide destruction with every formation.