User blog comment:Yarrghman/Seth, the Sand Mage/@comment-4593717-20130404152314/@comment-6353331-20130405061249

Other than the stun on Q (which is somewhat difficult to attain), W is Seth's only hard CC. I wanted its cost to represent that. For a bit of a comparison, look at 's. Even at low Sand (or health in Vlad's case), you can still cast it- but being at a higher percentage wastes potential output for further spells. Its cooldown doesn't change between levels, because the magic damage Whirlpool applies procs Abrasion.