EDIT: Thanks for answering and not yelling at me for being dumb. Good to see the community not hating each other's existence for once :)
[slayer-jinx-unamused]
His passive changes his ult.
So now if he gets all 3 stacks of his passive on you his ult does TRUE DAMAGE
Meaning no longer 1k magic dmg+300 or so true damage. IT MEANS 1K TRUE DAMAGE
From what I can tell, if he procs his passive on a target, they become "Deconstructed" and "Researched", and his ult will deal true damage to them. Also his Q no longer shows the range to enemies, which is a pretty big deal in dodging, etc.
That's about all I got
Q's made to be more of a last hitting tool with mana regen + enemies can't see the splitting point anymore Ult gains a new Research mechanic for consistent true damage
His ult has the potential to do full true damage on targets, and his Q indicator (the triangle thing where it splits) is no longer visible to enemies. I'm pretty sure they said from the very beginning that Vel was going to have the lowest scope changes out of the big 6.
They just wanted him to have a more distinct identity than the likes of Xerath or Ziggs. In this case he has the potential to do some massive damage from afar if he researches the champion first.
We don't know for sure yet. Wait like 1-3 days for it to go on PBE and check Surrenderat20.com
When he ults and it procs his passive, enemies become "researched." Researched enemies takes true damage from his ult, but it no longer procs his passive on them.
