Elden Ring: Where To Get Greyoll's Roar

gamespot.com:

So you're wanting to get Greyoll's Roar in Elden Ring, but you're not quite sure how to make it happen. Well, it's not going to be a quick feat unless you're packing quite a high level and some excellent weaponry. But don't worry--just because it may take a while doesn't mean it's a difficult spell to obtain, and we'll walk you through how to make it happen in this guide.

Greyoll's Roar is an incantation that requires 28 Faith and 17 Arcane to cast. It emits a massive AOE roar that deals heavy damage to foes while reducing their damage by 20% and increasing the damage they take by 10% for a full 60 seconds. This makes it an extremely powerful option against bosses.

Greyoll's Roar's item description reads:

Superior incantation of Dragon Communion. Channels the power of Greyoll, the Elder Dragon. An incantation of legendary status. Transforms caster into a dragon to emit a terrible roar, reducing the attack power and defense of nearby enemies. Greyoll was the mother of all dragons, dwarfing all who stood before her like a looming mountain.

Greyoll's Roar can be purchased from the Dragon Communion Altar using a Dragon Heart, but you must defeat the great dragon Greyoll before being able to unlock it. Greyoll is found just outside of Fort Faroth in eastern Caelid.

Defeating Greyoll is likely to take you a while if you're not leveled way up yet. Luckily, you can stay up against her tail far behind her massive body and whittle away at his health without putting yourself in any real danger. You may get bored wailing away for so long (it could take upwards of 15 minutes), but as an added bonus, you can also score a sweet chunk of runes for your time. Once she's deceased, you'll be able to go score Greyoll's Roar at the Dragon