Summary
The Leviathan is the UNSC Navy's flagship, commanded by Admiral Micheal Stanforth. It has seen many engagements including the battle of Sigma Octanus IV, and was present during the Fall of Reach.
The Levitahan was one of some 20 Marathon Class cruisers, sporting several MAC guns.
On page 162 in Halo: Fall Of Reach, the Leviathan is classified as a Cruiser, but on page 212, its a carrier-class.