Transmutus Primaris

When the first of the new breed of Adeptus Astartes were revealed by the reanimated 13th Primarch and the Heretek Archmagos of Mars, the Iron Templars were initially resistant and highly suspicious of the new warriors, regarding them as a perversion of the God-Emperor's holy work. Chapter Command, including Grand Master Adelwulf rebuffed the undead Primarch's offers of fresh reinforcements, confident that these "new marines" were the work of the dark powers or worse.
It wasn't until the 132nd year of the 42nd Millennium that a change in the stance of the Templars towards the Primaris began.
At that time, High Chaplain Sigifrid of the The Reclusiam approached the Grand Master with astonishing news: he had been given a vision from the Emperor himself, and shown a vast field of Primaris brethren in the livery of the Iron Templars, charging a mass of heretics and burning all before them. When the ancient Grand Master confirmed this vision with his High Chaplain, he prepared himself with grim resolve to be the first to undergo the procedure that would make him a Primaris Marine. Adelwulf was not eager for more power or strength, for his faith in the Emperor had gifted him these in abundance. But he was a leader who led from the front, whose armored boot was forever the first to strike soil in any attack undertaken by his crusade.
However, tragedy struck when the Apothecarion and Martian Magos Biologis began the procedure on the Grand Master. During the operation, Adelwulf's body began to reject the Primaris organs, his piously pure body resistant to the tampering of the Martian Adepts. What should have been his apotheosis to Primaris-hood, was the old Grand Master's doom. 
When Master Ricohard of the MCXI Crusade ascended to the rank of Grand Master, he decreed that until the Apothecarion was confident in their ability to perform the Primaris procedure, the Templars would not cross the Rubicon Primaris. He mourned the loss of his mentor Adelwulf, and the knowledge and skill that died with him. 
Newly promoted Master Theothelm begged his Grand Master to reconsider, citing the now-dead Sigifrid's vision. Ricohard refused for several years, before his own High Chaplain Cato advised letting the Templars of a single crusade attempt the transformation if they were willing. The Grand Master finally relented, and Theothelm was the first to volunteer his MCXX Crusade for the procedure, letting the Apothecarion operate on him before his men to ensure their technique had been refined sufficiently. 
Thus, Master Theothelm became the first Iron Templar to cross the Rubicon Primaris. His men followed, receiving the Primaris organs and enhanced Mark X armor. Some faltered in their apotheosis, succumbing to the same failure of health that the elderly Adelwulf had. Most, however, joined the ranks of the new crusade with stronger bodies, weapons, and equipment.