The Norns placed the giant Kartag as a guardian over the Twilight Well. This dark well is the twin of the mystic well in Asgard. When waters from the two are mixed, great mystic energies are produced, which Odin can use to restore his own power.
Kartag is pretty tough. He claims to have guarded the well successfully for two thousand centuries (200,000 years), killing each and every intruder. In the very well itself, Kartag nearly drowns Thor. But all of this is a test, set up by the Norns to see if Thor and his kind were worthy of their assistance.
The Norns then temporarily release Kartag, so that he can aid Thor in resisting Mangog‘s attempt to destroy Odin and all of Asgard.
In battle Kartag likes to throw giant rocks. At the site of the Twilight Well, he also has demons at his command.