Jenna that is extremely aggressive tail rot and needs to be amputates ASAP. See if you can get to an emergency vet tomorrow, tell them that this has happened in just 2 weeks, from the time you first noticed the dark spot that we now know was a wound. The infection can quickly travel up in to her body + sepsis will kill her. There is no other treatment that will stop it. I know it sounds scary but beardies usually do very well with amputations and bounce back pretty fast.