Secret Area in Stormveil Castle: Elden Ring
You have to go to Stormveil Main gate grace.
Follow this path.
And you will arrive at Rampart tower grace.
After that, exit the room and turn left towards birds.
You will need to go straight and turn left up the stairs.
Jump over (not too far).
Then jump down and hug the wall going north.
Now you have to pause and look at the easter egg here.
Jump down twice.
Then jump down and fall through the ground.
After that, you need to follow the path.
Kill Crucible knight.
And get aspects of the Crucible: Horns.
You have to set the Incantation info.
Follow the path to get back.
Then go to any grace and memorize the spell.
Next, you will need to set any memory slot to Incantation.
Wield any sacred seal.
Up to choose Incantation, R1 to use.
Down to choose Mana, square refill Mana.
Horns can break shields and stances.