-Viable top laner:
Eh, you can take him top but his kit was mostly made for the jungle. His q makes it very easy to take camps like krugs and raptors and his e while a heal is one of the best ganking abilities because you can go through places where the enemy won't really expect it unless they know who they're playing against.
-The way his passive works is the earlier in the game you are in, the less your passive orbs give. let say if you have 3 kills at 10 minutes, your passive bar is probably only half way filled while if you get your first kill at 20 minutes, your passive bar instantly fills up. also, If you are 1v1ing someone and you die, you DON'T get the orbs from attacking them unless you manage to disengage long enough to absorb the orbs.
Basically, you don't really have a passive before you get your form, so your early game is weaker than other junglers but once you get your form and considering how fed you are, you can become a pretty good carry, especially if you go darkin which I think is still stronger than assassin form because you still do damage with sustain. If you're behind, I would always recommend going darkin form because you provide cc with your w and if you're low health, you can just ult the tank and be full HP again. But if you're significantly ahead and you get blue form, I wouldn't have a problem with it, but if they have a bunch of tanks such as sej jg and sion top then i would wait and go darkin form because then you shred both tanks and squishies as darkin, it really depends how fast you think you can close out the game.
By the way, if you spent all game killing ranged champions and your bar is almost filled with blue orbs, and you hit an enemy melee champion to top it off, you will get darkin form.
Build:
Darkin (Red)- 
or 
(personal preferrence), 
Assassin (Blue)- or , 
or 
Boots are your own personal preference, I would either recommend resistance boots (Tabi or Merc Treads) on Darkin form and Mobility boots on shadow form.
(Builds are whatever you need in the CURRENT game that youre in, adapt your build to the enemy, this is just an example build i usually always go on these forms.)
. But I have some questions and struggles.