can morde's ult be stopped by cc in its animation?

i was playing a game yesterday as morde and i had a hecarim running at me and he was using his e? the ability that makes him run fast and i pressed my r on him as he was colliding with me and t put my ult on cd but we never went to the death realm. i dont play morde so im not sure how he works and was wondering if this is normal or maybe a bug?