Well, using the system over here, specifically the Vorpal Tribble system, you come out with a CR between 16 and 22, depending on how much you think stuff is worth. Now, I'm no expert, but I would say that this would be a decent challange for an unoptomized group of level 20s, maybe a bit weak, so I would say CR 19-20. That last one is very approximate though, use it at your own risk.