In his days as a pupil, Taelus trained under Vorazun, and looked up to her as a master. He firmly believed in the cause she would often speak publically about, that the Aiur protoss were slowly eroding Nerazim culture, and had no respect for their traditions. He also held an ideal that actions were far more important than words, which made him devise a plan to take action against the Aiur Protoss.
Taelus and four dark templar followers took control of the Daelaam Citadel. Their intention was to force the Aiur protoss to fight and kill them, and in doing so provoke violence between the two races. The resulting violence would inspire the Nerazim to throw the Aiur protoss off of Shakuras and allow the Nerazim to live as they had lived without losing their culture to the Aiur Protoss.
However, his former master caught wind of this plot, and personally with Mohandar tried to talk reason into Taelus. Tealus saw her as a traitor to the very cause she fought for, and refused to back down. With no other choice, Vorazun and Mohandar pursued him into the main Citadel chambers. They opted to fight rather than let Taelus provoke an incident between the two peoples.
Taelus's men were disarmed using nonlethal means, but Taelus impaled and killed Mohandar. Angered, Vorazun engaged him in one-on-one combat, killing Taelus with a warp scythe to the chest. In his last moments, Vorazun said she understood why he did what he did, and promised to protect their culture. In spite of his deeds, Taelus was given an honorable funeral beside Mohandar.