" Strangest of all are the suggestions that Tír is linked to a sister world, variously known as Geb, Jörd, Gaia – or even Terra, a name akin to that of our world, Tír. Manifold are the references to this sister world in the legends of Tír, notably those of the warrior races such as Fir Bolg and Shee. In both traditions the two worlds are spoken of as twins – as if worlds like infants, might share a single cosmic birth. Communication and even passage between worlds is said to be possible through Dromenon or the power of the Fáil. This same passage is said to have allowed the seeds of war to be carried from one world to the other, though such history is uncertain and largely denied. Most intriguing is the suggestion that the fates of the two worlds might also be entwined, as is occasionally seen, even dramatically so, in living twins. The implications are unknown, perhaps even unknowable. Yet one is tempted to question if the answers to the afflictions of one world might be discovered in the struggles and tribulations of the other? – Ussha De Danaan: last High Architect of Ossierel "

― Frank P. Ryan , The Sword of Feimhin (Three Powers, #3)