Peace Chess

Paco Shako (Esperanto for 'Peace Chess') is a chess variant where 'captured' pieces don't disappear, but coexist peacefully ('unite') on the same square, embracing each other. Such hugging pairs can be moved by both pieces (i.e. they move their own piece, which then drags the other along), but only to empty squares. When you 'capture' a union, this releases your own piece from the embrace, the newly arriving piece taking its place, and you must then move away the released piece in the same turn. This way you can get chains of releases. Kings cannot initiate embraces, and the player who manages to hug the opponent's King wins.

I made a web demo for this variant, where you can play it against the computer, at

http://hgm.nubati.net/paco

Apart from what you mentioned here, do all FIDE rules apply? Promotion? En passant?

Is there stalemate, perhaps when you have no legal move escaping from direct threats on the King?

Promotion and e.p. apply, even for pieces in a union. You must even promote when you are dragged to last rank by the opponent. For e.p. capture of a union the entire union is moved one step back, and then captured. Pawns can also be dragged to their own back-rank, and have a double-push there (which then is subject to e.p. capture).

There is no check rule: you win by embracing the King, which is the equivalent of King capture. In human games this usually occurs through some long release chain that the losing player overlooked. Stalemate is not possible in practice.

You would usually resign when checkmated. Unless you can capture the King. What in FIDE would be checkmate is very much like 'brink mate' in Shogi, and the release chain to capture the King is like tsume.

I now composed four mate puzzles (hug puzzles?) based on this idea:

http://hgm.nubati.net/paco/puzzle1.html

http://hgm.nubati.net/paco/puzzle2.html  ("The Diamond")

http://hgm.nubati.net/paco/puzzle3.html  ("The Dark Knight")

http://hgm.nubati.net/paco/puzzle4.html  ("The Triangle")

I do not understand the rules.

Try to play against the computer when in some moves I used the hint option.

After some losses I won but I do not understand the game

 1. e4 Nf6 2. e5 d5 3. exf6 Qd6 4. Nf3 Qxh2 5. Rxh2 P*h4 6. RQg1 Qe2 7. PNg4 Qxe7 8. Bxg4 N*xf2 9. ?xe7 Q*c5 10. Nc6 Bb5 11. Kd8 Qxd5 12. undefinede6 QPd7 13. QPd5 QPd7 14. QPd5 QPd7 15. QPd5 QPd7 16. QPd5 QPd7 17. QPd5 ?xd5 18. Q*d7 RQh1 19. Qxd8

I understand that after 5.Rxh2 I get additional move with the pawn and play h4

I understand that practically 6.RQg1 is a move of the black queen.

Later the game continue 7.PNg4(black move) Qxe7(white move)

8,Bxg4(black move) Nxf2(black move because knight is released and need to move)

Now I am not sure if I tried to play illegal move and got ?xe7 when the pawn simply disappeared from the board and I got additional queen move and played Qc5

I do not understand the ? and I do not understand the undefined later and I wonder if there is a bug that allow me to play illegal moves.

I think also that notation is not good,

1)It should have number only before move that white play and not before move that black play(otherwise it is confusing) and if a player play 2 moves in a row it should be written as 

A(B)

2)It should have the first piece as the piece of the player who move(meaning Q(R)g1 and not R(Q)g1

It means pgn to start with 1.e4 Nf6 2.e5 d5 3.exf6 Qd6 4.Nf3 Qxh2 5.Rxh2(Ph4) Q(R)g1 6.Qe2 N(P)g4

7.Qxe7 Bxg4(Nxf2) now 8.?xe7 (Qxc5) that take both queen and pawn at e7 out of the board initially and later put the queen at c5 is not clear to me.

I tried to repeat the game

Note that I could not play h4 by myself and needed the hint to do it and I could not reproduce
?xe7 Q*c5

It seems not to know about the 50 move rule or about the repetition rule.

Here is the game that seems never to be finished and I did not think a lot about the moves because I do not think I understand much about the game.

1. e4 Nf6 2. e5 d5 3. exf6 Qd6 4. Nf3 Qxh2 5. Rxh2 P*h4 6. RQg1 Qe2 7. PNg4 d4 8. Bxg4 N*xf2 9. Bf4 PNe4 10. Nbd2 Nd7 11. O-O-O dxe4 12. N*xd2 Rxd2 13. N*xe4 P*c6 14. RQe3 cxd7 15. Kd8 Bxc7 16. PNc5 dxc5 17. P*c6 RNc4 18. RQd3 RQe3 19. RQd3 RQe3 20. RQd3 RQe3 21. RQd3 RQe3 22. RQd3 RQe3 23. RQd3 RQe3 24. RQd3 RQe3 25. RQd3 RQe3 26. RQd3 RQe3 27. RQd3 RQe3 28. RQd3 RQe3 29. RQd3 RQe3 30. RQd3 RQe3 31. RQd3 RQe3 32. RQd3 RQe3 33. RQd3 RQe3 34. RQd3 RQe3 35. RQd3 RQe3 36. RQd3 RQe3 37. RQd3 RQe3 38. RQd3 RQe3 39. RQd3 RQe3 40. RQd3 RQe3 41. RQd3 RQe3 42. RQd3 RQe3 43. RQd3 RQe3 44. RQd3 RQe3 45. RQd3 RQe3 46. RQd3 RQe3 47. RQd3 RQe3 48. RQd3 RQe3 49. RQd3 RQe3 50. RQd3 RQe3 51. RQd3 RQe3 52. RQd3 RQe3 53. RQd3 RQe3 54. RQd3 RQe3 55. RQd3 RQe3 56. RQd3 RQe3 57. RQd3 RQe3 58. RQd3 RQe3 59. RQd3 RQe3 60. RQd3 RQe3 61. RQd3 RQe3 62. RQd3 RQe3 63. RQd3 RQe3 64. RQd3 RQe3 65. RQd3 RQe3 66. RQd3 RQe3 67. RQd3 RQe3 68. RQd3 RQe3 69. RQd3 RQe3 70. RQd3 RQe3 71. RQd3 RQe3 72. RQd3 RQe3 73. RQd3 RQe3 74. RQd3 RQe3 75. RQd3 RQe3 76. RQd3 RQe3 77. RQd3 RQe3 78. RQd3 RQe3 79. RQd3 RQe3 80. RQd3 RQe3 81. RQd3 RQe3 82. RQd3 RQe3 83. RQd3 RQe3 84. RQd3 RQe3 85. RQd3 RQe3 86. RQd3 RQe3 87. RQd3 RQe3 88. RQd3 RQe3 89. RQd3 RQe3 90. RQd3 RQe3 91. RQd3 RQe3 92. RQd3 RQe3 93. RQd3 RQe3 94. RQd3 RQe3 95. RQd3 RQe3 96. RQd3 RQe3 97. RQd3 RQe3 98. RQd3 RQe3 99. RQd3 RQe3 100. RQd3 RQe3 101. RQd3 RQe3 102. RQd3 RQe3

Paco-Ŝako can be implemented to forbid undoing union moves. But both make sense, on the official playing website they allow it (and I disallow it on my website). Consider the game draw if three repetitions, and play some other move if you don't wanna draw.

Here is a game just played against the bot (default level, probably rather weak but playing some good moves).  1. e4 Nf6 2. d4 e6 3. Bg5 Nxe4 4. Bxd8 BQg5 5. Nf3 f6 6. Nxg5 B*xf6 7. NQc1 Nc3 8. Bb4 Bb5 9. Bxc3 O-O 10. h5 NBd5 11. PNd2 c4 12. PNf3 PNf4 13. PNh3 cxd5 14. N*xc7 PBf3 15. Rxc1 N*d3 16. RQd2 RQc2 17. RQd2 Qc2 18. NPc6 Qc5 19. Nxc6 P*xb5 20. Qe7 bxc6 21. N*xe7 a4 22. QNf5 axb5 23. B*xc6 N*e7 24. QNh4 QNf4 25. PBg4 PNh4 26. PNf3 PBg5 27. gxf6 P*xg5 28. B*xf4 N*e2 29. Kh1 Nxf4 30. B*xd2 Q*xf2 31. Nxf4 Q*d6 32. PQg1 g4 33. NNg2 Nxc6 34. B*e4 Kf7 35. Bg6 Kg7 36. Qe7 Kg8 37. Qf8 PQf2 38. Qxg8#

And some links: https://vchess.club/#/variants/Pacosako , https://pacoplay.com/