While initially, a devout follower of the Golden Order, Miquella forsook it to try and find something different, all because he wanted to cure his sister of the abhorrent Scarlet Rot she harbored. To this end, he created unalloyed gold needles, with which the rot could be kept at bay. This, however, was not a unique effect to just rot but could stem the influence of any outer god, even the Frenzied Flame.
Perhaps out of defiance of his destiny and his birthright, Miquella chose to grow a new Erdtree watered with his own blood, which would eventually become the Haligtree. He not only took in those who did not belong anywhere else, like the Albinaurics, but also fashioned himself a strange cocoon to try and rid himself of his eternal youth. It's here that his destiny shifts greatly.
While in his cocoon, Mohg arrived and stole his half-brother thanks to the power of another outer god, one who could give power to cursed blood, and the same god that Mohg draws power from to create Bloodflame.
Mohg wanted to awaken his half-brother, even going so far as to get his cultist followers to give their blood to him, but it was to no avail. At some point much later, Mogh is defeated by the Tarnished, and Miquella continues to sit in his cocoon, at least in the current place and time.
