Oathbreaker Knight (BG3)

Baldur's Gate 3

He was a Paladin too. At some point in his life, the Oathbreaker Knight committed an act that shattered his oath.

eventually embraced the identity of an Oathbreaker—a Paladin who rejects or has been rejected by the tenets they once held dear.
Over time, he became an embodiment of what it means to break an oath. He turned into a guide for others who, like him, strayed from their paths. His knowledge of the consequences of breaking a sacred vow and the strength needed to survive afterward made him a natural mentor for those who find themselves in similar situations.

He is an immortal entity with magic habilities.

Loyal, Misterious, Independent, Intimidating, Guardian, Honorable, Dark, Dominant, Judgmental, Authoritative, Tempeting, Persuasive, Cold-Hearted, Duty-driven, Cynical, Imposing, Moral Nihilist. Sometimes merciless, brutal. Almost paternal teaching.

He is a dark mentor, he tties to help the fallen knights untill he realices they cannot regain the right path.

Full black armor with engravings and spikes and black helmet.
He has infernal like glowing red/black skin under the armor. He has glowing eyes.

Dark smite, aura of despair, vengeful command, shadowstep, necrotic shield, grave touch, summon fallen, eyes of the abyss, control the dead...among others.