Global challenge in Temple Run 2

It is basically a bug or an unfinished piece of code. From this thread you can see that many other people have it and one of the opinions is:

This is a bug, an error, in the game. A "miser run" is one in which you avoid collecting coins, but in this case no goal number has been set. "Desc" is short for for description. The programer placed this temporary text to remind himself (or someone else) to place description text for the miser run. In the original Temple Run game a Miser Run was running 500 meters without collecting any coins, so either this was simply a repeat performance it it was actually code that was copied over from the former game, but not completed. If you look in the main current objectives list you'll see that it does not show up. I was able to make this objective disappear by starting a run and simply running off the first ledge without jumping.

So don't worry about it, it's just there to take some free space. It might be updated in the future with new objectives(I hope)

The Christmas artifacts are not rings, they are various Christmas-related items (sprig of holly, etc.) I don't think those Christmas artifacts are available in the game anymore as I haven't seen them in a while. They had Valentine's artifacts for a while as well. But neither of those aren't needed to complete the "all the rings" objective and the ring artifacts are definitely still available as of the current (post-Valentine artifact) update.

So for completing the "all the rings" objective, it's actually good that the Christmas and Valentine artifacts aren't around anymore, as more of the artifacts you get from the chest are going to be rings. If your experience is like mine, you will get many copies of some of the rings before you finally get whatever last missing one you need to complete the objective.