As a part tank character, I would expect at least one tank skill or effect. Your kit guides the player to want to initiate headfirst, but you can't expect to survive such initiations just by giving Rah'kel tanky stats to compensate. I know you can burrow as an escape tool but there's nothing other than your base stats to sustain you in the fight once you initiate. Was the purpose of this champion to initiate and overpower the enemy rather than survive an assault?
I like how you made his skills scale because I honestly couldn't decide if I would build Rha'Kel with more focus on AD or AP. With an AP build, the Q has insane potential and it stacks well with the ulti and passive, but with an AD focus your E becomes more effective, especially with enough cooldown, which will allow you to refresh the armor shred over and over, though only in a very small time frame of 0.4 seconds (I assume it would be only refreshed since you didn't say it can be stacked). Either way the repeated slow effect, plus with a Phage/Frozen Mallet, would mean impossible escape and striking that fear into an enemy player is what void creatures are all about.
Now this is just for myself, but does the name Rha'kel have some linguistic meaning or is it just a random combination that sounded nice in your head?