I think i'm just going to echo what everyone else has said about this champion. Rhaast is 100 times better than Shadow Assassin. Their mobility is honestly not far enough from each to other to matter and in this meta the hard CC Rhaast provides makes him so much more viable. Percent damage on Rhaasts abilities gives him a ton more reliable damage, especially vs tanks. The flat damage his abilities do as Assassin feels so weak because the base damage is so low and could really be augmented by doing percent missing HP making them executes, Adding bonus magic damage to the abilities instead of his AA's, or just straight raising the damage. If the idea was to make Shadow Assassin scarier vs squishy teams it failed. Rhaast has built in healing on his abilities which lets him sustain in fights and can make him really hard to kill coupled with DD. His ult as Assassin does literally no damage. Sure it resets your passive but you get a few empowered AA's that you wont typically even be able to utilize since you'll just get kited to hell because no cc and survivability with a sub-par gap closer. As Rhaast however, if used on the right target can give you a massive heal and chunk a tank along with giving time for your other CD's to come back and get even more healing.
The champ has a lot of potential and as an Assassin main it saddened me to see bruiser form be so much better. Another thing to note is his name. "Shadow Kayn" or "Shadow Assassin Kayn" sounds so much better than just generic Shadow Assassin.