A total reality collapse is an event that occurs to a -verse or different types of large cosmological structures when its underlying system of logic becomes unable to support causality and continued existence. They can happen for a variety of reasons; for example, a paradox that escalates to the level of contradicting everything at once will cause one, as will direct influence by a cosmic entity capable of overwriting logic to a format where nothing exists or can ever happen.
Generally, once such a collapse begins, there is almost no way to stop it, and it was generally never around to be stopped due to its nature of erasing the timeline and shredding apart causality in the process.
In some cases, a collapse may leave behind or otherwise partially return to existence an entity from the deleted causality - in effect, something or someone that may have existed in the -verse "before" the collapse and was promptly affected so as to never have been around at all. The Unlich is one of an uncountable number of examples of this.