What IS an "otherwhen"?

At its most basic, an "Otherwhen" is a divergent history alternate reality. However, it has an extreme difference between itself and reality. This difference is so vast that the parallel universe cannot sustain itself, and will rapidly collapse. The danger in this occurs if the inhabitants of an Otherwhen realise their existance is fleeting. Such beings may well do anything to stop themselves from being negated. This usually involves attempting to alter its parent reality in a way that lines up with its own distorted history. Should they succeed, the divergent reality will overwrite the parent reality, and the parent reality will in turn become the unsustainable Otherwhen.