Kronor is the moon of Sirius. It has a very cold atmosphere and contains a number of military complexes, bases and prisons used by Mental's Horde. It also houses a fleet of warships that the Alliance used to invade Sirius, and the most powerful laser cannon known in the universe, which can easily penetrate even Sirius' shield.
Kronor is the 6th chapter of Serious Sam 2, and it's the only game it appears in. Not much else is known about Kronor. The chapter itself is rather short but fairly difficult, as two of the levels have a special "freezing" system (meaning the player will be constantly damaged if his/her temperature is too low). You may also be able to see what looks like the top part of Austrailia, the rest being behind a bunch of clouds. you can see it during the cutscene when the laser hits the shield of Sirius.
Kronor has a total of 5 levels, 1 of which is a boss fight:
Hugo is the boss of Kronor. It is hidden inside a boss box, and emerges when Sam enters the hangar it lies within. Then, it proceeds to chase Sam all the way to a helipad, where Sam defeats it using the helicopter's missiles.