One of the new additions in Persona 5 Royal that the original game didn't feature is Will Seeds. These collectibles are hidden within each Palace you'll encounter and collecting all three Will Seeds within a palace will fuse them together to reward you with a brand new accessory.
If you happen to miss any Will Seeds, they can later be purchased from Jose's shop, but they're pretty expensive. It's easier and cheaper to simply collect them yourself, not to mention you'll regain some SP each time you grab one. Just below you'll find a full guide on where to find each of the Will Seeds in Madarame's Palace.
Will Seeds are always found behind overgrown doors and Mona will notify you when you are close to one.
Red Vanity Seed Location
On Museum 2F, you will eventually find yourself trapped in a gallery where electric barriers go up as soon as you enter the area. You have to deactivate the barriers by jumping on top of one of the blocks and grapple to the next area to press the hidden button.
Once this is done, go up the stairs on the eastern side of the map, defeat the powerful shadow, then grapple from the central area on this floor to the walkway above.
Move along the walkway and then dropdown. You'll find a treasure chest behind you, and the Will Seed door in front of you. Head on in and grab your prize.
Green Vanity Seed Location
Once you reach the Treasure Hall Gallery, you will drop down to a new floor and see the next Will Seed door locked behind a barrier. In order to unlock this door, head north and then go down the path on your right and use the grapple hook there (it's the location shown on the map above). This will take you back up to the previous floor where you can loot a treasure chest and press a button.
Pressing the button will remove the barrier from in front of the Will Seed door so you can head back down and claim it. Afterward, head north to continue the palace infiltration.
Blue Vanity Seed Location
You can find your final Will Seed shortly after discovering the treasure's location. On the eastern side of the map there is a vent to the right of a large painting, travel through here and you'll find a chest.
After looting the chest, head into the open balcony and use your grapple hook to go into the rafters. Jump across the beams and travel through another vent to reach a room where you can drop down to the floor below once more.
Head right and open the locked door by interacting with the button, then save in the northernmost safe room you discover here.
Next, leave the safe room and head back the way you came, towards the left of the map where there is a dead end. You'll find the final Will Seed door at this dead end, guarded by a powerful shadow.
The Awakened God mini-boss that you have to fight is weak to Wind and Nuclear attacks, so make the most of your Atom Ring and Mona's Wind attacks to hit its weakness and do All-Out Attacks whenever possible. It can inflict some pretty annoying status ailments, such as Brain Wash, so keep on top of healing. (If for some reason you struggle and wipe, that's why we recommended that you save beforehand.)
Once defeated, you can head inside the room and grab your final Will Seed, which will cause them all to fuse into the Crystal of Vanity accessory that gives the wearer the Brush of Vanity Skill.