Minecraft Legends piglin keys are one of the trickiest resources to collect in the game, partially because they don't show up on screen like all the ores in your resource meters. To keep upgrading your villages and structures you'll need a steady supply of Minecraft Legends gold, and a great way to get it is from piglin chests found at each horde's outposts. The only way to open these chests is with a piglin key, the game will warn you, but it only gives you vague hints on where you're supposed to find them. If you have no idea where to start, the answer will always be: slaying more piglins.
Where to find piglin keys
The best place to find piglin keys is killing piglins roaming the Overworld. According to the in-game journal on the far right tab of your songbook, "though any piglin may be found carrying them, bosses and tougher units, especially those found roaming the world, are the best place to look."
In my experience, taking out multiple horde outposts in a row, along with fighting any piglins I encounter while en route between them, keeps me in steady enough supply that I can open the piglin chests at outposts without trouble. Definitely keep an eye out for elephant-like Lava Launchers, rolling Pigmadillos, and the chain arm wielding Portal Guards as you roam the world.
As of launch, there is no way to know if you have a piglin key currently in your inventory, which is quite frustrating. Though other special items like Power Towers will indicate in the songbook how many you've built, the piglin key icon in the songbook does not. There's no actual inventory screen in Minecraft Legends, so you have no way to know whether you've got a piglin key on you other than to just try opening a chest.
After you've got your hands on some piglin gold, make sure you know how to find diamonds in Minecraft Legends too.