When paired with a mana cost reducing card, particularly Omniscience in Historic, we can infinitely play Acererak from our hand
While there are cheaper options in mono-black like Raise Dead, Fell Horseman gets the edge in my opinion by also being a creature you can play later if needed
Oh, and a shout out to Magus of the Wheel which was my #13 initially but had to get pulled off in the numbers, sorry
You can mana sink five into hin at instant speed and make your Thopters grow into size equal to your hand
We liked the life-gain rider as this set uses gaining life mechanically to represent being happy, but Set Design wanted to distance the card converted mana costwise from Return to Nature ( Throne of Eldraine 's Naturalize variant), so the life gain was changed to card draw (white, like all colors, gets cantrips)