After a long search, Ronan acquired the Orb, but after having seen its true destructive capabilities, betrayed Thanos and chose to take the Orb's power for himself. Using his new-found power, he attacked Xandar, only to be defeated and killed by the Guardians of the Galaxy.

Is Ronan the son of Thanos?

Ultimate Marvel

The Ultimate version of Ronan the Accuser is the son of Thanos, and is a part of his empire. He is ultimately defeated by the Thing.

What happened to Ronan at the end of Guardians of the Galaxy?

When Thanos destroyed the Nova Corps and decimated Xandar to retrieve the Power Stone, he accidentally rewarded his former ally turned enemy Ronan. While he was killed at the end of Guardians of the Galaxy, Kree mass murderer Ronan actually won thanks to Avengers: Infinity War.