Boros Moxite runs a wide array of small creatures (Thraben Inspector, Glint Hawk, Sneaky Snacker) that Arms of Hadar's -2/-2 sweeps effectively, making it a strong sideboard addition against their go-wide plan. Nihil Spellbomb helps interact with their Sneaky Snacker/Pursue the Past graveyard shenanigans while replacing itself. Echoing Decay is redundant with the stronger sweep effect from Arms of Hadar since it only hits one name at a time, and Debt to the Kami is a slow answer to a deck that rebuilds its board quickly. Keep Cast Down and Snuff Out to handle their bigger threats and rely on Counterspell to stop key pieces like Journey to Nowhere or Melded Moxite from resolving.