I don’t have experience with this situation, but to me the tail looks necrotic. I would would get them to the vet asap.
As said above,
Have you accidentally shut in a door?
Or is it living with another?
I agree that your crestie should be seen by a reptile vet.
If shut in door or bitten by another Crestie it will most likely come off.
Seen this happen a few times and the Crestie lost the end of tail. Infact even recently was contacted by someone about a black end as the tail got shut in a door. The end fell off and still had its tail just with no tip!
Infact my KJ has his tip gone due to a bite when young, it just died and fell off.
If spreading though, the vet will just get the Crestie to drop its tail, you can do it yourself by holding onto the tail, or pinching the base of the tail until it throws it off.
If not spreading just leave it be and the dead end will fall off anyway.
As said above if it’s an injury it will likely solve itself. But keep an eye on it
If you don’t believe an injury has occurred, is there any chance the Crestie is a cappuccino Crestie. They have black tip tails and I believe it has spread as it gets older. But they are quite new to me so can’t say for sure.
They are living with their sibling but I have been wanting to separate them because the other always picks on him.
Well there ya go, it would appear the other has grabbed his/her tail!
Separate now,
If fighting they should’ve been separated straight away.
If damaged already then shedding there would be a pain anyway.
Unless both females they shouldn’t be kept together anyway, and even females can fight! Regardless of the injury, with the fighting they need to be parted today.
I agree but I think the original problem is stuck shed.