This page of the guide to Elden Ring explains where you can find the entrance to the Cathedral of Dragon Communion. This is one of the two main places where you can buy dragon incantations.
The Cathedral of Dragon Communion is located in Caelid. It's in the southern part of the map. The closest site of grace is the Caelid Highway South - you can virtually see the tall cathedral tower from here. Simply go towards it.
Below, on the map you can see the exact location of the cathedral.
A boss named Decaying Ekzykes will stand in your way. It is a large dragon that you can safely bypass on a mount. Go straight towards the cathedral, and the boss will forget about you after a while. This is a very strong opponent, intended for high-level characters.
Upon entering the Cathedral of Dragon Communion, you have to watch out for a single enemy who's hiding on the right. This is an exiled knight armed with shields and a large sword. He will attack you as soon as you enter the cathedral. When you defeat him, you'll be safe. You can then use the new site of grace.
This is the altar where you can buy dragon incantations.
Cathedral of Dragon Communion - what does it give?
The Cathedral of Dragon Communion is one of the two main places where you can purchase dragon incantations. Simply interact with the altar. You can buy incantations with dragon hearts that you obtain after defeating dragons in the game world. The Cathedral of Dragon Communion will expand its offer as you kill more dragons. Come back here every once in a while. The Cathedral of Dragon Communion has more incantations than the Church of Dragon Communion.