Havel the Rock
Lore
Havel the Rock was once a bishop and a close friend and battlefield compatriot of Lord Gwyn's. He served on the battlefield alongside Gwyn, leading a group of men in armor carved from solid rock. These warriors wore a ring to help them use their heavy armor effectively, and to show faith in their leader. Havel is found locked in a watchtower in the Undead Burg, long hollowed.
Havel wielded the Great Magic Barrier miracle in order to combat his sworn enemy, Seath the Scaleless. This miracle can be found on the path to Ash Lake. Its placement further cements Havel's role in the war against the dragons, which occurred in the vast expanse of archtrees below Lordran.
Theories
Havel and Ash Lake
Why He was Imprisoned
Sources
NEXT: Hawkeye Gough
The "Havel and Ash Lake" theory says that "the weapon they [Havel and his warriors] used was a Dragon Tooth" but I don't think that's 100% true. - Havel's set states that it's "worn by Havel the Rock's warriors" but the Dragon Tooth description says that it's the "Legendary great hammer of Havel the Rock" which implies it's a unique (Legendary) weapon that only he wielded. Havel's warriors may have used any kind of weapon, such as Zweihanders or Claymores, but they definitely did not wield Dragon Tooth.