I was playing Vladimir when the enemy Morgana ulted. As you would think, I used my W to try to block it and continue the chase. However, I was suprised when I was stunned even though I was untargetable and unable to hit.
Its a Tether. Same thing is applied for 
drain as well.
A Tether type ability will persist since when it connected you where targetable. You have to break it by getting away from them.
Her ult is a SOUL Shackle... it grabs the soul of the enemy. They cannot escape it by going "invulnerable" Hourglass doesn't stop it, Fizz's playful Trickster doesn't stop it, ect. You can only break it by getting out of the range of it.
The same for any DOT or delayed damage. Pool doesn't stop it once it has attached.
That's the intended behaviour.
Becoming untargetable doesn't help against something which has already hit you.
