Many worlds exist. The one you are from is not unique, nor is it special. You cannot surpass your limits, and you must obey laws set in place by dead gods aeons before the concept of you even started to exist. These are called mundane worlds, only viewed by the omniscient, and never acted upon.

There are worlds, however, where gods have allowed some capacity for growth. They are the worlds that many above lay their eyes upon. Here men may roam, unrestricted by certain rules, depending on what has been allowed. These worlds are called extraordinary worlds.

The presence of the supernatural can cause stagnation in the local peoples, however, so on occasion a human from the mundane gets the opportunity to roam these extraordinary worlds. The men and women of the mundane have coined a term for this.

Isekai.

This directly translates to "other world", but for the purpose of clarity it shall be used here in reference to the event of a resident of the mundane to be allowed on the extraordinary. The reasons for an Isekai are varied, as are the gods in charge of orchestrating them, but when one catches the eyes of one above, they may be brought here.

Here is a point outside of reality obeying nothing and everything. Here is where strong figures are brought to entertain the ones above.

Here is the tournament of Isekai.

Being in an Isekai is not required to participate, just the collective will of the gods above worlds.

"Sit ye here and commence. The eyes of the many are watching."