Oh wow this looks really great and authentic! Nicely done~

Thank you! I'm glad it seems convincing ^^

This can make for a very scary commander

Well, possible, although nobody would want a commander that won't be scary in some way at least. Also he has no evasion or protection so he is easily removed. Also you can block him easily as well. I kinda would compare him to Atraxa in an infect. Every turn she stacks poison counters on everyone and Yhnir deals potential commanser damage just that he needs to have dealt damage before to do that while Atraxa just needs to stick around until then.

If you can keep him on the field, and/or give him some +1/+1 counters, that's a very short clock the enemy is looking at. Thank god he doesn't have flying.

I agree, flying would clearly have been too much. Although he can still get removed easily if not given some kind of protection as well.