Redrock Sentinel

Land sacrifice as resource conversion, packaged into a wall. The activated ability spends and a land to hand back two things at once: a fresh card and a Treasure that immediately refunds a chunk of the cost, which is the wrinkle that keeps the loop from collapsing into pure card disadvantage. You feed it a land, you get a card and a mana rock that stands in for the land you gave up, so the tempo hit is softer than "sacrifice a permanent" usually implies. The result is a grinding value piece rather than an explosive one: per activation, a tap that gates it to once a turn without a way to untap it, and a Defender body committed to sitting back. The ability runs at instant speed, so the Treasure and the card can arrive on an opponent's end step, letting you convert a spare land into resources on their clock rather than yours. The 2/4 frame does exactly what a Defender should, buying the turns the engine needs to matter, and the artifact typing means it profits from artifact synergies (through the Treasures it manufactures) while sidestepping removal that only hits nonartifact permanents. This earns its keep in decks that treat lands as fuel rather than a fixed asset: strategies happy to fill a graveyard with spent lands, sacrifice shells that want repeated fodder, or slow control that would rather draw two effective cards from a stalled board than deploy a threat.