Rolling Cactus

They sit in the Underground Desert looking like scenery until something disturbs them: a pickaxe, a stray projectile, or the ground being mined out from under them. Then they roll and do not stop for anything, piercing every enemy and player in the line. Hitting one with a pickaxe starts it moving rather than breaking the tile, so every cactus in a world is single use and the ones you place yourself behave the same way.

Which way it goes is decided by whether its horizontal coordinate is odd or even, not by the slope it is sitting on. It will happily roll uphill and ignore a downward incline. The only thing that overrides the choice is a wall on one side. That makes them awkward to aim deliberately and easy to trigger by accident from above.

Dropping onto a lower ledge keeps its forward momentum and scatters spikes in random directions on the way. Slamming into a vertical wall does the same but destroys the cactus. Speed changes none of this: one that has rolled a single tile hits exactly as hard as one at full pace, so being close to it when it starts is no safer than being far away.

Because they hit hard and are trivially triggered, they are one of the standard tools for killing a town NPC when you need a drop that only comes off one.