How does the poet use allusion in the poem, The Fairy Reel?

The lightning tree is an allusion to Norse mythology, in which the god Odin was hung from a sacred ash tree. This tree was believed to connect the lower and upper worlds, in much the same way as the speaker themself is caught in a place between worlds. The concept of being nailed to the tree also alludes to the crucifixion of Christian mythology. In both cases, a sacrifice was made preceding a rebirth. In the case of Odin, the sacrifice is a direct trade for sacred knowledge. The poem suggests that the speaker undergoes a challenging period of inner transformation in exchange for new knowledge of this other world. Ultimately, however, this knowledge becomes a hindrance for the speaker as they reach the end of their life.