I wrote this in a Google Doc since it's much more convenient.
https://docs.google.com/document/d/1qiEwoxxPavXDi1REfHi_HWXxjOvXv_ewxxo3giegnoA/edit?usp=sharing
Art was NOT made by me. Feedback would be greatly appreciated!
TheFirstGokun2 votesReal great concept, I like the idea of a more systematic assassin playstyle. Excellent ideas across the board with Contract and Abduct (both the active and the passive), but I do have a few questions.
Is Bistir Ranged or Melee? This affects how his assassinations work and also how he uses Phantom + Blink of an Eye.
About Phantom, why does it scale with AP? What kind of damage does it do? Why is the invisibility so short? I'd like to know more about your reasoning behind this ability.
I'll be honest, it kind of seems like you added all the Final Destination synergies because you didn't know how to synergize the entire kit cohesively. Nevertheless, they're sort of interesting on their own, and I can see the potential combos. Again, it would help if you wrote down your reasoning for the individual abilities.
That's all the bad things I can say about this concept, and believe me, I tried! Excellent work!
Reviewer, away! 
Gushis1 votesThank you kindly for the feedback!
Bistir is a melee champion
Phantom does magic damage and it scales with AP for, frankly, fun builds. Otherwise no reason at all. It does magic damage. The inviaibility is short since colliding with a unit resets the invis timer (can't do same unit twice for the same cast)
I added final destination procs to add depth to the kit and give Bistir players a choice for each situation. Are you going to use double Q into R for extra burst and dash? Will you use Q into W to make a swift excape? Maybe Q W for slow, making it easy to hit Q into R? There are just so many possibilities for this champion, and that's why I added the interaction.
I agree, I would make it a habit to explain my reasoning behind every ability
Thanks so much again for the feedback, Very solid and very helpful
I'll be posting my remaining 11 concepts over the following month(s), I hope you would like those too! Heads up, they contain teasers, in game pics of kit, 100+ voiced lines, and more
Gushis1 votesAlso if you want some feedback on any of your concepts just hmu
TheFirstGokun1 votesThanks, I'm actually working on one right now. Will tell you when its ready :D