playstrategy.org/EkLkpHA0bQc7

19 moves, rook underpromotion

playstrategy.org/FseCwwfJ/p1#85
Knight in 87 ply, opponent didn't use the chance to underpromote as well to delay mate by 2 ply 😂
Also, a very thematic captcha 😂

1

playstrategy.org/BgRFndEWElRM
Squeezing down to 73ply

1

Entries for Weekly Challenge 25 has closed. Thanks to all users who participated.

The winner is @e-pluszak who won with an underpromotion to a Knight in 73 ply in both of the following games: playstrategy.org/pxwNNKFc playstrategy.org/BgRFndEW
The first was a discovered attack, which was a clever way round the challenge, enabling promotion to any piece. The second was a genuine mate with a Knight, which is worth highlighting.

Users are still welcome to attempt this challenge and post answers here, but the winner has been fixed. The next challenge can be found here: playstrategy.org/forum/weekly-challenges/weekly-challenge-26-draw-the-lines