Guardians get various bonuses to make them stronger based on the game difficulty. Defeating most guardians will start a Leviathan Parade Opportunity situation. Guardians have an immense amount of health and should only be engaged mid-late game. While one can just stumble over one during exploration, they can also be found with the help of the Curator Enclave.
They are marked with a skull icon both in the fleet power box and on the galactic map, and the exact strength number can only be seen with debug mode on.
They can be found in special predetermined systems and rival moons in size and mass. Guardians are extremely powerful Spaceborne aliens added in various DLCs.