Wow, its actually balanced! That is a tricky theme to make work.
I really like the icons, the work you put in shows.
Is it possible to trigger his ult early by clicking it?
That's pretty much all I've got
Spells are creative, and very well balanced in my opinion. I like that his strength is greator than his agility.
I have one question. Have you considered reducing the movespeed bonus in the ult a little to add a zero pathing aspect for the spell? Late game with chasing items he should catch most people, but creeps can still prevent him from catching people. I wouldn't give him flying pathing (from your balancing skillz, I'd say you agree that that would be imba) but when the ground gets sufficiently cluttered, a 522 night stalker still gets bogged down. Considering you've only got a short revive duration, it might be worthwhile.
I can understand leaving it as-is though. If an enemy is smart enough to creep/image block purposefully, he should get away