Only killing nonblack creatures does make it a bit narrow, but it makes up for it by allowing you to tap out to develop your board and still have interaction up to prevent your opponent from developing theirs
Since it it colorless to cast, every deck can use it as part of their top end
We havent seen any of the other Infinity Stones yet, and, as was unveiled at PAX West this weekend, it doesnt look like we will be for quite some time
There's creaturefall on The Great Henge, enchantmentfall on Verduran Enchantress, and, of course, artifactfall. Artifact fall cards love seeing all those trinkety tokens like Treasure, Food, and Clues lying around, as well as all the myr and Thopters of the world
Similarly to Seven Dwarves from Throne of Eldraine, Nazgl allows players to break a formats deckbuilding rules