Comments posted to our Dark Souls 3 Wiki
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"Pontiff Sulyvahn says in his dialogue that he imprisoned a God of the old royalty, Gwyndolin, son of Lord Gwyn (the old royal)."

Dialogue? Does Sulyvahn speak in the game? Never seen it happen and cannot find anything about it...
By far one of the easiest bosses if you're playing melee. Just charge in, and hit his tail until he teleports. From there on, repeat until victory.
Edit: He is extremely weak to frost as well. Irythill Straight sword +3 deals huge damage to him. I beat him no sweat with that weapon SL 55.
For those of you planning on summoning another for the fight make sure to dodge the soul masses after you kill him before gesturing. I have seen far to many hosts die because of that little bit of because of that little bit of derpness.