I hate to say it but I feel this would be over powered. For instance, my army normally has gore pilgrims extending the range of the portal of skulls to 36" making my opponent re-roll all successful casting. For unbinding I have 3 slaughterpriests, flesh hounds, bronze rune (auto-dispel), collar of khorne (gives hero unbinding), not to mention the blood tithe auto dispelling. An opponent is very unlikely to get ANY magic cast if you can have multiple unbind attempts especially after they have re-rolled for the portal.
I know Khorne isnt uber competitive but I have been doing alright and thats mostly down to shutting down magic (and gore pilgrims of course )