The Witch of Morin. Definitely the Witch of Morin. A thousand years ago, give or take a century, she was a woman who invented and openly practiced witchcraft (playing D&D, a "witch" was a homebrew class I made that involved mixing magical potions that gave you access to spells and prolonged your life, but also aged you horribly) who lived in the grand city of Morin. She was exceptionally talented and powerful with magic, and she had a coven of followers.
People grew fearful of her (in a time where the Elves had not yet migrated to the mannish lands and magic had not become a "well-known" concept), and they cast her out, burned some of her followers at the stake, and drove her away.
Her brews had given her long life and her spells, but they also aged her into a haggard old woman and they also fueled her paranoia that the entire city was out to get her and destroy her for her practice. Women brave and knowledgeable enough to find and enter her lair were initiated by her and taught the ways of witchcraft, and were ultimately corrupted by her ever-growing evil influence. She taught witches to destroy and corrupt, and she held no greater contempt than for the people of Morin, especially its royal bloodline, who exiled her.
About a century or two later, she grew so paranoid and so crazy that she amassed her followers, made somewhat of an army, and together they set fire to Morin. The entire city was burned to the ground, and the once beautiful castle was permanently charred black. What very few survivors there were hated the Witch of Morin and her coven, and they rebuilt their city from the bottom up.
This day, centuries later, the Witch of Morin is alive and somewhat well in her lair, where none dare to tread nowadays. Her followers were scattered after the attack, and they created cults of witchcraft that turned the weak-minded initiates into hags. The Witch of Morin, completely consumed and driven to madness, is the oldeset witch to never have made the transformation, as her seething hatred has fueled her longevity along with her legendary potions.
She is too insane to ever reach her full power of ages past, and instead keeps to her lair, where she sits and plots empty plans, worries about Morin's people rising against her, and spends her time bitterly cursing the people of Morin (not magically, just swearing and cussing), the joys of life, and goodness.
When the heroes found her at the request of Morin's royalty (after their city has made a near total recovery, the city is still permanently scarred), they were expecting a god-tier sorceress with incredible power and danger, and instead they found a decrepit, aggressive, insane, and pitiful old woman fueled by small inklings of rage and hatred. I always make comparisons to Lord Gwyn from Dark Souls and his state during the final battle: A once powerful being reduced to a miserable existence. Still formidable and incredibly powerful, but insane and washed out.