Although there's no way to get unlimited FP permanently in Elden Ring, there is a way to enjoy the benefits of limitless FP for short spans of time. If you've seen clips online of players melting down bosses with a single stream of magic, their FP bar not dropping at all, this is how they're doing it. All it takes is the Flask of Wondrous Physick and the Cerulean Hidden Tear. Here's how to get it.
Where to Find the Cerulean Hidden Tear
The Cerulean Hidden Tear can be mixed into the Flask of Wondrous Physick to prevent the use of FP when using spells, but only for around 10 seconds at a time. To get another use out of the Flask, you'll have to rest at a Site of Grace.
As is typically the case with Crystal Tears, you'll receive the Cerulean Hidden Tear by defeating an Erdtree Guardian at one of the many Minor Erdtrees scattered around the Lands Between. The Cerulean Hidden Tear will drop when you manage to defeat the Ulcerated Tree Spirit, which guards the Minor Erdtree in Mt. Gelmer.
To reach this Tree Spirit, head to the Road of Inquiry Site of Grace. From there proceed directly east until you reach the base of the Minor Erdtree, where you'll find the Ulcerated Tree Spirit.