We've played it as allowing a save when first cast. Without that, it would be way too powerful. Even so, as written, it is a bit strong. The DC to escape is nearly imposible for any low-level or low CR creature to make.
I would suggest lowering it to a flat 15, or making it equal to some number (10 or 12?) + 1/2 caster level if you want it to remain useful at later levels.