Maybe Im just very stupid and missing something.
But if I have 972 AP, my E deals 140 plus 6% (+34%) of max health.
Against a dummy with 10,000 HP and 100 MR (50% damage reduction) that should be 2070 damage. But it deals 307 damage. (00:15)
Against the red buff with 3360 HP and 10 MR (~10% damage reduction) it should deal ~1300 damage but its 527 damage. (00:52)
Apart from that the damage seems to be inconsistent? Bought some Magic Pen against the Dummy and one time with W -> Q -> E, my E dealt 566 (02:22), the trys right after dealt 526 damage. (before and after 02:22)
In the end I just spammed non empowered Es and dealt 425, 400 and 440 damage (02:56) (Im not sure if this can be the result of one of the new runes but I guess the things above shouldnt)
Video for reference (very sry that I didnt have time to cut it, just check the timestamps)