Obdurate Prince was a traitor of unknown origin mastering his own plans of stellar domination at the 35th Millenium. His warbands and human armies made extensive use of psykers to subvert local populations, using a particularly horrific method of propagating widespread neuro-hysteria that could turn the people of a world against their lords in a single bloody night.
The Ultramarines Chapter was appointed to annihilate the traitor and his minions. Due to lack of time it was the only way to counter the enemy's plan – swift strike to the gathering of psykers who were the source of the unrest. Space Marines of the Ultramarines Chapter used their own Librarians to home in on the Obdurate Prince’s psyker conclaves before sending in Terminator Squads via teleportation.
The goal was achieved, the mission completed – all enemy psykers were destroyed. It was first major change in using composition of a Strike Force Ultra - all infantry in Strike Force Ultra formation were to be equipped with suits of Tactical Dreadnought Armour.