So after playing both Shadow Kayn and Rhaast. I have to say Rhaast just feels much more powerful, not only that but also way more viable. I played shadow Kayn first and must say his ult just feels really underwhelming. The whole concept of the ult is amazing and I like what it is but as shadow Kayn I really think it should be more of an execute. Having the base damage but a percentage of the missing health. Rhaast get percent maximum for damage plus healing so I do think that shadow Kayn should that the percent missing extra damage. But then you could say ohh that'd be way too op. Well look at it like this, Rhaast can be very tanky but also have lots of damage through items like titantic hydra and such. Plus he does percentage max which is good for tanks but he can also stick really well to the back line through the ult as well. making him a good tank/bruiser. Shadow Kayn is suppose to be more of the high dps fighter/assassin so being able to deal the damage then ult them to make sure the kill is secured would be a very satisfying feeling.
Basically just add a percentage missing health extra damage or just have the damage be based on percentage missing health (opposite Rhaast).
This would make it similar to Zed but I think that would be a good thing as Zed is technically Kayn "leader" so having that shadow similarity would make sense lore wise plus it would make the ult very satisfying to use. Not that it isn't the damage on shadow kayn's ult just feels really dinky and out of place in his kit.