The “Show your humanity” puzzle can be found in the Ringed City Streets bonfire, in the room that can be found right after the shortcut has been opened. On the far side of the wall, you’ll find some statues and one broken statue with writing on the wall that states “Show your humanity”.
The secret to opening this puzzle is to use the Chameleon spell or to use a White Branch consumable item. But it has to be in a specific place, namely, while in the swamp outside the area.
Once you go all the way back to the room and go near the message, a ladder will drop and you will be able to go into a secret area. This area will contain two Ringed Knights, one with a sword and the other with a spear, along with a summoner in the area. It’s quite a difficult fight to do all three at once, so try and kill the Ringed Knights one by one and then kill off the summoner.
You can then proceed to drop down into an area containing a Dragonhead Greatshield and the Purging Monument that Lapp wanted you to find. You can then go talk to him to continue his questline, afterwards he will spawn in the Shared Graves area.