An evil baron who served King Mark of Cornwall.
He knew of the affair between Tristan and Isolde, and he conspired with two other barons to expose it to Mark.
For a long time, Mark wavered between believing the barons and believing Tristan – and alternately banishing from his court one or the other. Finally, while laying another trap for the lovers, Denoalen was attacked and slain by Tristan.
Source
Tristan | Béroul, late 12th century