Double Diamond is quite a bright stallion — metaphorically and literally. Not only he has white fur that can glow in the dark, but he was born with an ever-increasing amount of knowledge in his mind. He doesn't grow his brain to do so, but his mind becomes deeper and deeper.

But with more knowledge comes a greater risk of becoming a magical conduit. And every year, more magic begins to fill him, opening him to terrible secrets that he will never forget.

With enough time, his intelligence became his ruin: the sheer force of that excessive magical energy burned his soul and broke his mind, driving him to insanity and despair.
Alas, poor psyker XD. Perhaps you could put yourself in the Emperor's service?

"Help, help, help me! I can't control all that power, it's not for me! It hurts! Aaaaaaaargh!"