When you’re playing on a Sorcerer and salvage Istel’s Grimoire from the seasonal questline, you will receive an unobtainable transmog for your focus!
Normally, the focus cannot be obtained in the game, because it’s not flagged as a world drop, but due to the way transmog works in Diablo 4, you can often salvage items with an identical appearance on specific classes, for transmog.
Here’s a screenshot of the focus. Courtesy of kristmate.