Illegal Position Contest!

this is truly illegal

#7766 is clearly illegal. But also simpler than its predecessors!

Here's what I'm guessing is the easy illegality explanation Arisktotle has in mind.

White's first retraction to uncheck the black king had to be -Bc7xb8. It had to be an uncapture, or else there had to be a black unit at c8 to block queen checks, to give Black a previous move. The capture on b8 or c8, added to the 14 captures by the white pawns, means White needs 15 captures, but Black is only missing 14 units.

Is legal!

Seems legal to me.

Legal.

gxf5 b7#

This is so people can see what #7771 looked like before EvinSung deleted it.

The g6 pawn must have originally started on the g-file. Rg7 needs to be set up before the bishops, but both bishops have to be set up before g7-g6 is played, so either Rg7 is impossible or one of the bishops cannot be set up.

This is an example of an illegal island. No matter what else is on the board and where, the structure in the upper right is illegal all by itself. I created several retro-problems with such islands. It's very frustrating to unwind a messy diagram and then discover there was an illegal island on the board all the time which you will never ever release. Watch out for those before you waste your time on constructing a proof game!

Nobody seemed interested in my easy position last time, so here is a slightly harder one.

Black has no possible last move.

Edit: Or last but one move if it's Black to play (FEN shoes White, but that can be ignored).

It's illegal because of the pawns, not the kings position.