While I didn't anticipate Rodolf Duskbringer and Markov Purifier receiving this much attention, I also couldn't expect Blood ArtistBlood Artist and Cruel CelebrantCruel Celebrant to be played in so few decks
Delve decks need redundancy like cheap cantrips and self-mill
The front side of Ballista Watcher is pretty bad: sure, you can pick off a random utility creature like Birds of Paradise, but you have to spend 3 mana and tap it, not great
Anyway, here are some alternative options if you're interested: Cathars' Crusade, Celestial Crusader, Battleground Geist, Sephara, Sky's Blade, Intangible Virtue, Obelisk of Urd, Coat of Arms
Each one is indicated by a chaos symbol to the left of the ability, though the symbol itself has no special rules meaning
Otherwise, its good enough to play for the +1 ability alone