Trapped inside that accursed watch for so long, living a false life… No, he hadn’t been living, he’d merely been existing, but with the opening of the watch he was finally free.
It had provided the perfect disguise, perhaps too perfect. He’d forgotten himself, forgotten the power he wielded, but no more. He remembered now, and he had everything he needed, even a TARDIS. Not his of course, but what did that matter? It would serve him regardless.
Locking himself inside he allowed his regeneration to proceed, gifting him with a new, young body. The Master was back in business