Four kongs plus a pair.
Four kongs and a pair, called Eighteen Arhats for the eighteen tiles involved. It is a limit hand in our rules. Each kong needs all four copies of a tile, so the hand consumes sixteen specific tiles, and each one is declared openly as you form it.
Effectively unbuildable by intent, and worth knowing about rather than chasing. Each kong gives a replacement draw, so a hand that stumbles into two kongs at least gets extra looks at the wall. Take those, but plan an ordinary hand around them.
Declaring kongs recklessly in the hope of reaching it. Every kong exposes information and removes flexibility from your hand. Two kongs is already unusual; four is a story you tell afterwards, not a plan.
As a limit hand it replaces every other row, including All Pungs and Kong Replacement. Each kong still draws a replacement tile that may finish the hand.
Can the kongs be concealed, exposed, or mixed?
Any mix. All four simply have to be kongs.
Why is it called Eighteen Arhats?
Four kongs and a pair is eighteen tiles, against the usual fourteen.
Has anyone actually made this?
It happens, but rarely enough that most players never see one.
Play Mahjong Winds
Every win shows its faan line by line, using the table on this page.