Background
Prior to beginning the Wanted! quest, Solus Dellagar was already widely known as the "murder mage", due to the fact that he was a mass murderer. He had managed to elude the authorities for so long due to his incredible magical abilities, as he had enough skill to be able to slow down the process of teleportation enough that he was literally in three places at once- the point from which he teleported, the Abyss and also the destination of his teleport spell. He was also capable of greatly increasing the damage that could be done by the Flames of Zamorak spell, allowing him to cripple even the strongest player. Fortunately, Solus was captured at some point by the Temple Knights and was kept their prisoner until recently.
Release into society
When the player originally joined the Temple Knights, Sir Tiffy Cashien had failed to go through the proper method of recruitment, and so in order for the player to progress up the ranks of Temple Knight, they were required to join the White Knights. In order to do this a crisis was needed, and so it was arranged for the release of Solus Dellagar in the hope that the player, along with the Temple Knights, would be able to recapture him.
Death
By using secret Temple Knight technology, the player was able to track Solus to the Kinshra headquarters and then the Zamorakian shrine in Varrock. Eventually the player managed to trap him at the rune esssence mine. A squad of Temple Knight archers arrived to take Solus into custody, but he was able to defeat them all in a single blow, leaving just the player to fight. Solus was eventually slain, leaving only his hat as proof of his death, and allowing the player to fully join the ranks of the Temple Knights