p.boxed
  | Every 20 moves, most empty squares freeze and isolated pieces vanish.

p
  | Ice Age is a 
  a(href="https://www.chessvariants.com/boardrules.dir/iceage.html")
    | chess variant
  |  invented by Köksal Karakus in 2000.
  | It can also be played on 
  a(href="https://brainking.com/en/GameRules?tp=100") brainking.com
  | .

figure.diagram-container
  .diagram
    | fen:rnbqkbnr/pppppppp/cccccccc/cccccccc/cccccccc/cccccccc/PPPPPPPP/RNBQKBNR:
  figcaption Deterministic initial position

p.
  If the board is filled with ice cubes, you can only move by capturing them.
  Ice cubes behave as normal pieces, which cannot move.

p.
  Just after the 20th black move (and before the 21st white one),
  "ice age is coming":
ul
  li all empty squares freeze, except those orthogonally surrounded by pieces.
  li all pieces which are not adjacent to any other piece (black or white) vanish.
p.
  If only one king remains, the other side loses.
  But if both disappeared, then the result is a draw.

figure.diagram-container
  .diagram.diag12
    | fen:2rq1rk1/3bpp2/1c1p1bP1/1p2n3/4P2P/1NN2P1c/PPPQ4/1K1R3R:
  .diagram.diag22
    | fen:cccq1rkc/cccbppcc/cccp1bPc/ccccnccc/ccccPccc/cNrccPcc/PPPQcccc/1K1Rcccc:
  figcaption Before and after 20...Rxc3

p.
  Then, every 20 moves ice age comes again: after move 40, 60, and so on.
  This condition excepted, the game follows FIDE chess rules, without draws
  on repetition: indeed, as the author notices, sometimes a win is possible
  by "waiting until the enemy king freezes".