This ring can be found in a locked chest at the Architect's Room in the Silverite Mine. It can only be opened with a key which can be acquired if Seranni is persuaded (Master Coercion is required) to help the Warden-Commander when she releases them from their cell during The Righteous Path quest.
See also EditRing of Mastery
What use is discipline, lack of skill?
When equipped with the Ring of Discipline, the character gains a bonus to constitution.