An Oath of Return is the sole method by which a fae may leave its place of significance, in the company of the dragon or sape that has sworn the oath. The oath is born of nature magic.
As mentioned in the oath, the one taking the oath is making a solemn vow to return. If this vow is broken by dying elsewhere in the world without having returned, the body of the one who has taken the oath will combust, and the fae will feel the pain of this fire as their own. The fae will drag the ashes of its partner back to its place of dominion, scatter them, and fade away, forgotten.
For this reason it is heavily recommended that only a dragon should take an oath of return, as a dragon will not die of old age before returning, and is usually far harder to kill than a sape.