Thanatosmon is a hypothetical Digimon whose design is implied to resemble MegaloGranids, the corrupt evolution of the Grandis Armor as a whole. When Martin was forcefully transformed into MegaloGrandis, he went insane with the sheer power he acquired, and left permanent damage to various universes due to his lack of any limitations on his powers. He was so drunken with rage and power that his rampage didn't end until his own continuously increasing power caused the armor to malfunction, before collapsing in a frozen wasteland, where the assignment for everyone in charge of taking care of the armor was to examine the helmet's circuitry to see what exactly had went wrong that caused the transformation to begin with. Out of the many bad things discovered in the helmet's memory log, Thanatosmon's existence is one of said things to have been as a direct result of MegaloGrandis's existence, but to this day, nobody has ever been able to find so much as a single trace that the Digimon in question was actually created by MegaloGrandis or not.
Based on the theory regarding Thanatosmon, it is believed that it's appearance resembles that of Bronze and Cobalt's take on Gaiamon, who did not actually exist as a legitimate Digimon, so Grandis took the decision to design what it would look like in his own image. Due to this, Gaiamon heavily resembled Grandis in terms of it's armor, which due to this, Thanatosmon is implied to look much like MegaloGrandis, to the point an alternative name for this Digimon is RustGaiamon, or, more fittingly, Gaiamon Destroyer Mode. According to research based on the logs found in MegaloGrandis's memory, Thanatosmon is an X-Antibody Digimon, and apparently doesn't resemble Gaiamon as much as was initially believed. Regardless, despite these notes, nobody can confirm or deny that this Digimon is somewhere waiting to be discovered, or when exactly this renegade monstrosity will awaken. Still, no further sightings or investigations of the various Digimon continuities seem to pin point where exactly this mysterious Digimon is located. Current research has been unable to identify the location, or the exact power, of this Digimon, so temporarily it is considered Super Ultimate Level until further data is revealed about this creature.
On a side note, Thanatosmon gets it's name in a similar fashion as MegaloGrandis. In Grandis's set of attacks, if they happened to be named after a specific god, the Grandis armor is associated with Gaia, the Greek Personification of Life. In MegaloGrandis's case, however, all of the attacks have had their names changed to focus on the god Thanatos, the Greek Personification of Death. The roles seem to also change primarily as well; Grandis was an armor set that used more Defensive based tactics to deal damage, while MegaloGrandis specialized in actual weaponry in which to slaughter enemies with, and had much more firepower in general, even after MegaloGrandis had stabilized after the initial first rampage in which it's power level could not be matched by any other being. Due to the situation, it is often believed Thanatosmon is a myth due to the fact if MegaloGrandis had indeed created it, then it would suffer the same problem that MegaloGrandis had initially; a severe power surge to the point it's power level temporarily surpasses every known Digimon, until finally stabilizing itself into a Super Ultimate Level Digimon.