Option Ossus was a plan by the Jedi Covenant to destroy the Sanctum of the Exalted. Jedi Master Lucien Draay ordered Feln to carry out the plan in 3963 BBY. Borjak and especially Haazen was against the idea.
Ultimately, Feln executed the plan, when he thought that Zayne Carrick's companions had come back to raid the Sanctum and detonated the charges. But the explosion also destroyed the village below. In anger, the other Feeorin turned on Feln and killed him.