Event horizon is complete bullshit. This should be the event horizon.
I believe there is no employee in Riot who studied physics. You don't get stunned if you touch event horizon. You can't see what's going on inside the event horizon from the outside and there is no way to communicate in and out. Therefore, it should just create a giant circle that impairs enemy outside from seeing inside or inside from seeing outside (even with wards or allied visions like graves w) It would be way more interesting because Veigar would be able to use it on enemy to make him difficult to dodge skillshots or use on teammate to make enemy difficult to land skillshots, not just placing a giant stun zone without counterplay.