Covering the entire back, the image seen here is of a devil figure that seems to be taking the halo off of what looks to be an angel.
Personally, I feel that the devil figure has been rendered a bit better than the angel figure.
In terms of shading and scale, it is well done. The hair, horns and overall face are very consistent and nicely inked. The hand and halo are also interesting to look at.
For some reason, the angel hair, face and hand do not look as consistent with the devil image. Not sure if that was done on purpose – but the tattoo as a whole is powerful (and a bit disturbing!).