I want to say it's too early to tell, but I honestly wouldn't be surprised if he saw nerfs next patch. His passive in particular seems extremely good; the %HP shred combined with the dual AP and level scaling makes it deadly to both squishies and tanks, and the fact that it's constantly refreshed in combat gives you very few openings to deal with him as a melee champion. You either kite him, or get run down and out-sustained, making for very boring and linear gameplay which was one of the drawbacks of old Morde.
The free magic pen on his E also seems out of place. Why is it necessary at all? To fill up his power budget? Surely you could have added something somewhere else if that's the case, maybe making his Q a bit more interesting instead of a generic af slam. Tbh I think he'd be better off without the free %pen, the ability is good enough without it.
Unlike everyone else, I don't find his ult that big a deal. It has a lot more counterplay than the rest of his kit, and it's definitely thematically fitting, so I'd be fine letting it go.