There is much speculation on what the Authority of Envy can do, but nothing definitive has appeared. Satella has displayed great power, and while any of those could be related to Envy, what she can do most closely resembles Yin magic. From shadows, to affecting time, to breaking through dimensions, everything but Unseen Hands could be directly seen as use of Yin. And half of the world during her time was engulfed in shadow.
What’s more, that shadow likely happened before she became the Witch of Envy. There are two things that point to this (imo).
1. Emilia’s first trial. As a child, Emilia discovered her vast mana, but when she started using it, was unable to control it. She froze Elior forest, and all within, for over a century. Satella, regarded as nearly Godlike, would have a far greater amount of mana. Enough to swallow half the world.
2. Beatrice. A Great Spirit, like Puck, created by Echidna, like Puck. And created for a purpose, but that purpose was not to guard the Archive (stated by the Echidna of the Trial during the last tea party). Taking the parallels of Emilia and Puck, an ice magic user of vast power and an ice spirit requiring vast mana, Beatrice too requires vast mana and, being a yin type, would be best suited for a yin mage. Her purpose, like Puck’s, was to provide mana control for someone who had too much to control. She was created for Satella.
Shamak, as its more powerful versions are used, can also not only create shadows, but create dimensions, or break through to others. During that time, 400 years ago, is likely when Subaru was unintentionally pulled through, finding himself with Satella in an endless, lonely night that she didn’t know how to stop.
To me she reads the "memories of the world", only for know the past, presents, and future events of natsuki subaru😈, because when she becomes omega, says, you will be ready for the inminente future natsuki subaru?😅