Their king, Yhwach, granted each of them power. He etched a letter into their souls, bestowing them with an ability with that letter, a Schrift. The Bleach Thousand-Year Blood War anime has shown ...

Bleach Thousand-Year Blood War has revealed ... Each member is granted a Schrift, a unique power by Yhwach. When they pass away their power returns to Yhwach, making him stronger.