At the time of the Fall at Caliban Fallen Lord Dceet, was known as Achilles the Grand Master of the 16th Company of Dark Angels. He had taken part in the Great Crusades but like many of his brethren he did not agree with the Lion's tactical decisions, his uncaring nature to those he should be protecting repulsed Achilles. When he spoke aloud about such things he was sent back to Caliban for home guard duty. After this most of the records are lost of Achilles rumours are seldom heard and those that are seem to be very far fetched.
One possibility was that on his return to Caliban Achilles pleaded to the Dark Lord Tzeentch to give him the power to defeat the Lion. This power may have caused the initial fight on Caliban with Achilles giving the order to attack those that would leave Caliban to help the Emperor so deep was his hatred that if he had to defend Caliban so would everyone else. As a gift to Achilles or Dceet's (as he was now known) his men were given psychic powers or sealed in their armour giving eternal servititude to Dceet and to Tzeentch.
Another is that when the Fall happened on a Caliban and the Battle Barge "Lions Fury" crashed into the planets surface from the stars, nearly all of Achilles company was wiped out, as the Battle Barge fell he made an alliance with Tzeentch, if he saved their lives he would offer his services to him. Many of the 16th Company were sealed within their armour, their bodies crumbling to dust, their souls now controlling the suits of power armour. The ones further away from the crash site were granted with immense psychic ability, that then protected them from the massive blast wave.
What is known is that the 16th Company marched as fast they could towards the Keep to stop the Lion from entering and attacking Luthor, they were to late and although the 16th Company prevented any other intruders the epic battle between the two leaders was already taking place. After their failure they were cast into the warp through space and time a few of the sorcerers were mutated beyond recognition, and Dceet's psychic powers grew even more over time. After hearing activity of the hated "Sons of the Lion" Dceet headed out to the Eastern Fringes, using his psychic powers to hide his true form.
"The Sons of the Lion how far you have fallen, the Lion the embodiment of pride, honour and courage and yet you hunt the night and stalk the shadows, how you have become the very thing you so despise." Fallen Lord Dceet.