This scroll is not a scroll to learn a spell but is used to buy the book Multiverse Lore (a skill) in the Mage quarter/tower in Tindrem.
The only location of this scroll is from all I know the final chest in the Sator dungeon. But: There is a second scroll named Portal Dimension Knowledge which also can be used to buy that book. And this scroll is much easier to buy (!) for diamonds worth 2 gold in Sarducaa. Ok, the vendor can be reached easily in a cave but you can't return the same way and have to find your way through or around a camp of heavy fighters.
The scrolls can also be exchanged for the book at the Tindrem mage tower. Aur has the same vendor as well. These skills allow you to actually use the Ether portal spell (obtainable in the cultist dungeon) if it's scribed into your spell book. The higher the skills are trained the more spirits your boxes retain when you leave the ether.
And additionally one needs 60 points in the skills Ether Lore (primary skill, buy the book for 150 silver coins at the Tindrem library) and Ether Portals (secondary skill) to be able to use a portal. This means you can learn Ether Portals with a fighter and cross a portal together with a mage inside of somebodies palisades.