The real-world inspiration is Celtic mythology. For shapeshifting, see the Welsh legend of Taliesin en.wikipedia.org/wiki/Taliesin , in particular the part where he is chased by Cerridwen
"All too soon he heard her fury and the sound of her pursuit. He turned himself into a hare on the land and she became a greyhound. He turned himself into a fish and jumped into a river: she then turned into an otter. He turned into a bird in the air, and in response she became a hawk."
The powers of prophecy displayed by bards and druids match the Vision power. Blessings and curses are more a matter of a geas - restrictions placed on a someone by a magician. The story of Cu Chulainn en.wikipedia.org/wiki/Cúchulainn has in its middle an example of a hero turning into a giant, warped hero ;
"The first warp-spasm seized Cúchulainn, and made him into a monstrous thing, hideous and shapeless, unheard of. His shanks and his joints, every knuckle and angle and organ from head to foot, shook like a tree in the flood or a reed in the stream. His body made a furious twist inside his skin, so that his feet and shins switched to the rear and his heels and calves switched to the front... On his head the temple-sinews stretched to the nape of his neck, each mighty, immense, measureless knob as big as the head of a month-old child... he sucked one eye so deep into his head that a wild crane couldn't probe it onto his cheek out of the depths of his skull; the other eye fell out along his cheek. His mouth weirdly distorted: his cheek peeled back from his jaws until the gullet appeared, his lungs and his liver flapped in his mouth and throat, his lower jaw struck the upper a lion-killing blow, and fiery flakes large as a ram's fleece reached his mouth from his throat... The hair of his head twisted like the tange of a red thornbush stuck in a gap; if a royal apple tree with all its kingly fruit were shaken above him, scarce an apple would reach the ground but each would be spiked on a bristle of his hair as it stood up on his scalp with rage."
His death is an example of a Death Prophecy (in Ars terms) in action - he will die if he eats dog meat, but the social custom of never refusing hospitality forces him to eat mystery meat with dire consequences.
Yes, in a "realistic" setting, any gruagachan could become mighty heroes like those who transformed Irish history in myth. Then again, if it wasn't for The Code, hermetic magi would totally remake Europe. Just one powerful magus as a court wizard could raise any royal heir into a King Arthur.
