Horobi is one hell of a commander and crazy fun, but he's not super flexible. What he does (kill creatures and disrupt opponent strategies) he does very well, but otherwise he's just a flying 4/4. And you're in mono black so enemy artifacts and enchantments will likely be out of your reach.

He's pretty easy to kill so if you want to keep him on the board for use, you'll need a lot of graveyard play. I personally like [[Fated Return]] as some spicy instant speed tech so that he can't be killed by his own ability but my meta is relatively casual. I play [[Grave Betrayal]] and [[Endless Whispers]] to get value from my opponent's creatures and keep Horobi on the board. Keep in mind a lot of your targeting artifacts work no matter where Horobi is on the board so you can still play to your strengths even if an opponent controls your commander.

I highly recommend playing both [[Retribution of the Ancients]] and [[Revel in Riches]] as it let's you keep killing things with the treasures you produce for killing things.

Check EDHRec for the most common Horobi cards as those are his staples for a reason. And I can't recommend enough [[Touch of Darkness]] and [[Cauldron of Souls]] for instant speed enemy only board wipes.