The Relentless is complicated because it makes heavy use of the keyholder
mechanic. There are three rooms, and you are expected to enter each room missing
certain letters. Solving the available puzzles in each room opens the doors
between the rooms, which lets you cross into them with a different set of
missing letters.

There currently isn't a way to represent "is missing certain letters" in our map
data or randomizer state. Instead, we use rooms to emulate knowing which letters
are available. There is a room for each of the three entrances, containing the
puzzles solvable with the expected missing letters. There's a room for each of
the inner pairs of rooms, representing what becomes available when one of the
doors is opened, and a room representing what is solvable when both doors are
opened.

This is all done with the expectation that you are always entering The
Relentless with the correct letters in the Control Center's keyholders. Because
of this, the warps to The Relentless are not randomizable. The Control Center
keywords that open these warps are also not randomizable. It'd be nice to find a
way to randomize this at a later point.

Also note that in order to keep this functioning properly, if the player
receives a letter item while in The Relentless, the mod should hold off on
adding it to the player's keyboard. We may want to overhaul how keyholders work
entirely and just have some kind of thing in the Archipelago client's global
state.