Treachery is the way of the Sith. Ulic Qel-Droma's capture on Coruscant had been engineered partly by his ally, the Sith sorceress Aleema Keto: she had ordered Mandalore to withdraw their forces at a critical moment. However, after Mandalore helped to rescue Ulic from Coruscant, he secretly told Ulic everything. Aleema's fate was sealed.
Feigning ignorance, Ulic continued with the Sith Brotherhood's plans. Using Naga Sadow's ancient battleship, the Corsair, they planned to lure the Republic fleet into the Cron Cluster and use the Corsair to destroy its suns, annihilating the fleet. Aleema took the Corsair into the Cron Cluster and activated the superweapon, but Ulic had deliberately not warned her of the consequences.
The blast annihilated the entire Cron Cluster in a supernova, sending a shockwave toward the Jedi world of Ossus. The Corsair itself was consumed, taking Aleema with it.