Malphite's Ult Rework

As a level 7 Malphite, I believe he is in desperate need of a rework (as hard as that is for me to accept). It is inevitable that he will be reworked in the Tank update in the future. Here is an idea that I had for his new ult-

Shield of Stone- Target a nearby allied champion to sink into the ground and turn into a circle of rock around the ally that shields them. It works similarly to Yasuo's Wind Wall. Malphite will block most projectiles and take only 80% damage from them. The ally inside the shield only take 20% of the damage. Malphite is targetable when in this form, but the ally inside is not targetable, and the ally inside takes no damage from auto attacks. The shield ends when Malphite takes 400/500/600 total damage, but if he does not take that much damage, the shield lasts for 5/6/7 seconds.

Please leave a comment/reply about any buffs or nerfs or general changes that this could use. This is my first concept that I have come up with so it may be a terrible idea, who knows? Just leave your suggestions, bye! (The name is definitely up for change!)