Urza's Saga are the remnants of devices and magical artifacts that Urza left behind to aid in the repairing of time rifts, many of which he himself created, should any appear.
It is said that Urza came upon a discovery of ancient, powerful artifacts and learned his skill by reverse engineering them. Through the manipulation and recreation of these artifacts he was able to create grand inventions, and became known among his people as Chief Artificer. Greater and greater inventions poured from his workshop until he discovered the ability to create what would be later known as Time Rifts. Historians believe that time rifts are the universe's natural backlash against deicide, the ripple an event like that causes in the fabric of the universe is so severe it creates a tear in the weave itself. Urza soon discovered that with the right skill, mental capacity, and enough arcanomechanical augmentation, he could re-weave the fabric as it stitched back together ever so slightly differently. Nature abhors a vacuum and so these time rifts were seen are nearly instantaneous events, and it is said that Urza was one of the great masters in manipulating time rifts. Although miraculous and amazing, Urza's time rift manipulation attracted the attention of other multiversal beings, and although much is lost in the years that followed, many a great multiverse war ravaged worlds. It is said there was a being who gained his power from the very time rifts themselves, subsuming entire universes, but little is known of this creature, and most simply assume it to be a myth.
At some point historians generally agree that Urza either led or was a heavy proponent in the effort now known as the "Great Mending," that cured the multiverse of time rifts.
Many of Urza's inventions still exist and it is unclear whether these devices working in concert would serve a greater purpose any longer. The older denizens of the multiverse know that Urza's grand creations lie scattered across the multiverse waiting to be discovered, holding both the secret to, as well as the cure for time rifts.
Though no time rifts have been seen or felt in millennia, rumors persist of a threat of a rift opening some few hundred years ago; perhaps that is why some of Urza's inventions have been resurfacing and Urza's saga now reveals itself.