The judgement passed on Odo by his people in 2372 for killing another Changeling, after which the Founders took away his shapeshifting abilities and left him in a humanoid body.
Odo's judgement by the Great Link takes place in the Deep Space Nine episode Broken Link. Odo falls ill and begins to lose control of his form, and Julian Bashir can do nothing. The Defiant takes him into the Gamma Quadrant, where the Female Changeling comes aboard and explains that the sickness was given to him in order to bring him home.
Odo had killed a Changeling aboard the Defiant in The Adversary, the first time one of his kind had harmed another. He enters the Link on the Founders' new homeworld, and during the wait Worf stops Elim Garak from firing the ship's weapons at the planet.
The Link returns Odo as a solid, with a human body and only his old face left as a reminder. He also carries away an impression that Chancellor Gowron is a Changeling. His abilities come back in The Begotten in 2373, and the true origin of his illness is revealed later.