Dark Souls 3 the Ringed City Show Your Humanity Puzzle is at the Ringed City Streets bonfire, in the room right after the shortcut door in unlocked. You can see some status at the far side of the wall. One of the statues is broken and is used to tripper the puzzle.
You need to use the Chameleon spell to open this puzzle or use the White Branch as an alternative. However, you need to use in a specific place, outside the swamp area.
Dark Souls 3 The Ringed City Show Your Humanity Puzzle Solution
Once you reach the other side of the room and reach the statue with the message. A ladder will drop down that will take you to a secret area. There are two ringed knights in the area, one has a sword while other has a spear. There is also a summoner area.
It is best to kill one knight at a time and then kill the summoner. Once you are able to defeat them all, you can drop down into an area where you will get Dragonhead Greatshield and the Purging Monument that Lapp wanted you to find.
Now speak with him and continue the questline, he will later spawn in the Shared Graves area.