The Inheritors of Destiny are those who were selected by Time to be one of the four pillars.
Stephos himself is the main support of Time, his keystone acts as a centrepiece. However, to keep his keystone from being the only thing protecting Time, he set that every so many years there would be four people who time would also lean upon.
Usually, only one or two pillars exist at a time, making it nearly impossible to break the seal on the magniverses. However, every few billion years, all four will exist at once.
After the time set by Stephos passes, the four will cease to be Inheritors, whether they are alive or dead. If an Inheritor travels to a future where he is no longer an Inheritor, he will still provide strength to his time while having no effect on the time he visits.
asterik * indicates past