Commission; The Riddles of the Raven. Ravenwing Command Squad Castor

When a Dark Angel initiate becomes a full battle-brother each hears the words:

"From this day forward you are a Dark Angel. Nothing else is of consequence."

From then on they are taught the stories of the spiral - legends and  myths that have permeated the Chapter since its founding. As a Battle-brother grows into his role, further mysteries are illuminated, colouring past stories they have learned with new meaning. If a battle brother is deemed worthy, he may join the ranks of the Elite second company, where he must contemplate the Riddles of the Raven. 

Unlike his previous tutelage the student faces a baffling array of contradictory 'truths' that must be interpreted and discussed with their squad leader. Although rare, a few of these riddles have found their way out into the wider imperium such as the riddle of the three Emperor's

Three Emperor's wanted to find out who was the wisest one. So they invited an old man, who took them into a dark room and said: "I will paint each one of you an Aquila, a star and a circle onto your forehead. When you walk out and you see at least one Aquila, raise your hands. The one who says what is on his own forehead first, wins." Then he painted only Aquila's on every one. When they went out everybody had their hands up and after a while one of them said: "I have an Aquila upon my head."

How did he know this?

Unlike the progressive linear understanding the stories of the chapter give him, the riddles are designed to disrupt the usual thought processes of an Astartes, opening one's perception beyond the constraints of time, space & logic. Although the initiate will be unaware of this, this disruption is designed to hunt beings that have traversed the limits of the logical universe and survive the maddening limbo-logic of the Emperyan.