A Candle Against the Darkness
The world is rife with dark, mysterious magic which can be obtained given the right knowledge. For example, a wizard who finds an eldritch secret in a dead archmage’s spell book might be infused with ancient magic, as might a character who solves a sphinx’s riddle or drinks from a magic fountain.
Legendary creatures, such as the medusa, powerful hags, and dragons, sometimes grace their allies with charms, and some explorers find themselves bearing the primeval magic of ancient, long-lost locations.
The gods themselves, although reclusive and largely unconcerned with the affairs of mortals have also been known to grant their blessings to particularly faithful or useful servants.
The magics obtained in this way is typically short-lived but invaluable to adventurers.