Whats all this talk of Euron being a failed apprentice and him having been an ex protégé to Bloodraven,that seems so out of left field. And would that not also suggest that there were various failed protégés?

In AFFC, Euron says “When I was a boy, I dreamt that I could fly…When I woke, I couldn’t…Perhaps we can fly. All of us. How will we ever know unless we leap from some tall tower?” and subsequently takes the name crow’s eye. That’s really, really close to Bran’s three-eyed crow dream, and we know from both Bran II of AGOT and Bran IV of ACOK that Bloodraven is recruiting “dreamers” to help fight the Heart of Winter/Great Other.

As for the other failed proteges, the imagery of them impaled on giant icicles in the Land of Always Winter suggests that most of them die in their sleep.