Complex sentence: The alchemist Mirewelt cannot predict the consequences when, out of sheer gratitude, he gives the three brothers a share in his magic powers, but his three gifts quickly prove to be all it takes to turn the social order in Bruges upside down.
Split into simpler sentences: The alchemist Mirewelt cannot predict the consequences when, out of sheer gratitude, he gives the three brothers a share in his magic powers. But his three gifts quickly prove to be all it takes to turn the social order in Bruges upside down.