Shouhai Mighty is a novel variant of three-player riichi mahjong. As the title suggests, the game is played Shouhai (less tiles than usually) with 13 tiles held during one's turn, and 12 tiles after discard.
But how do you play with less tiles than normal without reworking the entire rules of mahjong? This is where the "Mighty" in Shouhai Mighty gets it's name. In the most basic terms:
Let's go over how this works with a few examples.
Here's an example tenpai of a hand in Shouhai Mighty (Pei is nukidora). In this case, the hand is waiting on the 4pin, 7pin, and the Hatsu. In the case that the Hatsu is discarded/drawn, the Mighty tile would be counted as either the 4 or 7pin, whichever would give the hand a higher value. In the case that the 4/7pin is drawn/discarded, the Mighty tile is counted as the Hatsu.
Here's another example:
This hand is tenpai, waiting on 1pin, 4pin, 6pin, 9pin and the Ton. Upon winning with the 6pin or 9pin the hand would score for Honitsu and Chiitoitsu, whereas 1pin and 4pin and the Ton would score Honitsu + Dabuton.
This hand is tenpai for chiitoitsu, however, the waiting tile is the Mighty tile. In this case, this is a chiitoitsu with an infinite wait, waiting on any tile in the game to complete. This shape with chiitoitsu can only be achieved by dismantling a winning chiitoitsu hand, and is deliberately sought after in order to score with open riichi, ippatsumo and chiitoitsu.
Some common rules regarding how the game is played:
※2 Han open riichi is allowed, and is required in the case of furiten.
※Depending on house rules, different amounts and types of Aka-dora will be utilized. Some examples are that all 5s are sometimes akadora, and sometimes non-standard 3 and 7 pin/sou Akadora are used. Sometimes, Akadora 5 man are utilized, and Pei is instead Yakuhai for all players.
※If the Mighty tile is standing in for a tile that is Dora, it counts as Dora. The exception to this rule is Akadora.
※The Pocchi Haku is normally used, and if it is drawn after declaring riichi, it can also act as an all-mighty tile, standing in for whatever winning tile you'd like. In some rulesets, this usage of the Pocchi Haku is only extended to the first draw (ippatsumo) after riichi.
※You cannot utilize the Mighty tile to mean you're using 5 of the same tile in your hand. Example:
In this case, because of the 8sou Ankan, this hand cannot win off of the 9sou, as doing so would mean your hand would have five 8sou. Therefore, it can only win off of 5-6-7sou and the Hatsu.
※You can make an Ankan after Riichi that would change the Yaku, but not the waits.
※Chiitoitsu with four of the same tile is allowed. In some cases, chiitoitsu with four of the same tile is called 'choro' and adds 1 han.
※By the same token, utilizing four of the same tile in your hand regardless of what melds they're being used in is similarly choro and adds 1 han. This counts even with the Mighty tile.
※You cannot overlook a winning tile after riichi. Doing so results in chombo.
※1 honba is 1000 points, and points are rounded up to to the next thousand interval.
※Play starts at 30,000 points with 40,000 point gaeshi. If oorasu ends with all players below 40,000 points, the game enters West (sudden death)
You can play Shouhai Mighty online using the fork of the pwmarcz mahjong autotable maintained by riichinomics here: https://autotable.riichi.moe/
It is not an officially supported game-mode in the setup, however, it is easily playable utilizing this tool. Types of Akadora can be specified, and if desired, 4-player Shouhai Mighty can be played, along with Washizu Shouhai Mighty.