Senna looks interesting, i love her design i like her R, Q and W but guys, c'mon.
E - The Curse of the Black Mist
Senna turns into a cloud of fog, gaining movement speed and camouflage for herself and nearby allies. Allies who come out of the fog are seen by enemies as ghostly phantoms that cannot be attacked until they attack or approach.
Like, i could get behind her making sort of a "Safe Zone" around herself that enemies can't attack her or her allies while outside of, that's pretty interesting but could definitely be frustrating and the fact that if you see the "Fog" you can't tell how many champions are hiding inside of it is definitely going to be frustrating, to add they can also launch their abilities from inside of it as well and that's pretty strong to. Why did you have to add that enemies that exit it are Untargetable? That's ridiculously strong, the whole ability is absurd and i'm getting Akali flash backs from it.
All i ask Riot Games is that you take a step back and rethink releasing a champion with that ability. A long time ago the champion Lux was supposed to have an ability that gave her allies Camouflage, it was scrapped because and i quote "Not every champion in the game is balanced around being Invisible." but your going to release a champion with an ability that does exactly that?
Again i love the rest of her kit she looks great and i always wanted a champion who satisfied my need for a global laser of death, but that E is terrifyingly powerful and it's going to be impossible to balance.
Edit: Forgot to mention that you can't CC her or deal damage to her to cancel her E, i imagine that this was done because she's always at the center so you always know where she is but it still makes the ability hard to play against because there isn't a counterplay to it. Your forced to go inside of it where her entire team might be waiting for you and i think that's too strong. If the Untargetability was gone and it could be canceled by dealing damage to or CCing senna, the ability would be frustrating but not overpowered this current interation is both frustrating and overpowered.