The Mythos is the namesake ship of a new class of Earthfleet battleship, that is a return to the old days of battleship design, focusing on size and firepower over speed and maneuverability. It is a big change from the Clayton Doctrine that was set down by Grand Admiral Mike Clayton after The Battle of the Oort Perimeter, where Earthfleet, going forward, would focus on smaller, more powerful, agile and faster ships, abandoning the ages old doctrine of using battleships and battle cruisers for primary Sol Sector defense. This doctrine came about after the incredible poor showing by Earthfleet's myriad of battleships, resulting in a near total loss of all frontline battleships, including the Nova Class, whereas only two, out of all of the shipkillers present at the battle were lost.
Even so, the Mythos isn't a complete return to the old battleship doctrine of times past, as this new class of battleship is more or less an oversized shipkiller designed with the purpose of taking on entire fleets, numbering upwards of 250 ships, and winning in much the same way its smaller cousins, the shipkillers are designed to take on ships many, many times their size and win. So in a sense, a Mythos class is little more than a super sized shipkiller. Only instead of killing just one ship at a time, it's designed to kill hundreds of them. The entire class, save only for its namesake ship, takes their names from characters out of mythology, both Roman, Greek, and several others.