Ronan was the son of Thanos, the ruler of the Acheron Empire. Ronan serves with the title of Accuser, a position with the duty to face and stop rebellions and disturbances across the Aceron Empire, the "Endless Resurgence". When the Fantastic Four, Dreamcatcher, and Tesseract were stranded on Pyx, the planet that Ronan had quelled a rebellion three months ago, Ronan attended to this by placing 'judgment' on the Four for assisting Acheron's enemies. He placed a judgment zone between the worlds of Pyx and Earth's New York City for him to 'judge' the Fantastic Four. Ronan proved too powerful for both the Fantastic Four and Seed Nineteen, including the all-powerful Tesseract. However, Reed Richards coordinated both his team and Seed Nineteen to weaken Ronan by having Threshold absorb the powers of her friends, allies and Ronan's cosmic power against the Accuser, allowing Ben Grimm to (painfully) wield Ronan's Ultimate Weapon and slam it against its owner, defeating him.
Superhumanly strong (lifting over 50 tons) and durable, Ronan depends on the staff-like Ultimate Weapon for much of his power.
The Ultimate Weapon can manipulate any energies it samples, protecting him from physical and energy attacks, allowing him to fire energy blasts, redirect teleportation beams, heal his wounds, or switch off other sources of energy, such as the Human Torch's flame. It is keyed to Ronan's genetic structure, and if another tries to wield it then it burns to the touch.