The L stone
The L stone is hidden in the chest inside the upper room in The Castle area.
Inside this room, you can see the chest at the end with a lot of eggs in between and a monster floating near the ceiling. Walking up to the chest and breaking any of the eggs will cause the monster to instantly rush and kill you. In order to avoid the eggs, you can't use any magic or have any item that uses magic, you also can't use any weapons (so select "nothing" under weapon in your inventory).
Alternatively, if you have the Rocket Boots you can fly over the eggs and land in front of the chest to claim it. But: Don't equip any items that summon allies or shoot projectiles; anything that destroys an egg will cause the monster to drop and attack, which kills you instantly. And don't fly into the monster!
Another method of avoiding monster's attacks is by obtaining Black magic grimoire and shortening spells cooldown using The Wishing Well and Pains Au Chocolat. After doing that, it is possible to block monster's movement by spamming Obsidian Wall spell.