If you're the sort of person who always has a good book at your side, this bag can serve that function quite literally. The Book of Vishanti, the tome which in Doctor Strange's world contains powerful positive magic spells and counterspells against dark magic, served as the model for this bag.
Filled with all sorts of good things, such as your laptop, a couple of magazines, an umbrella, some snacks, your sling ring, this bag holds a lot. With the symbol of the Sanctum Sanctorum embossed in gold on the cover, no one will doubt your mystical abilities. You may want to brush up on your Sanskrit, though. Or, at least, keep Google Translate handy.
To convert to a backpack: Detach the end of the strap closest to the buckle. Remove the buckle portion of the strap and the slider. Thread the attached end of the strap through the two D-rings on the back of the bag. Reattach the buckle to the strap and clip to side ring.