Wither Trap (Nether Stars)

Unlike everything else here, a wither trap doesn't spawn anything on its own. You summon the wither yourself by stacking three wither skeleton skulls on a T-shaped soul sand block. This is really a reusable, safe kill chamber plus a skull supply chain, not a passive spawner.

Wither skeleton skulls only drop rarely from wither skeletons in Nether fortresses, which makes farming those skeletons (sometimes with a spawner, if you're lucky enough to find one) the actual bottleneck on how often you can even trigger this trap. Three skulls per summon, so stock up before your first attempt.

The containment chamber matters more than the summoning ritual itself. The wither's spawn explosion wrecks most blocks in a radius, so the standard design summons it deep inside a pit lined with obsidian, well below the surface, so the explosion is contained instead of cratering your build or letting the wither escape.

The wither is briefly invulnerable right after spawning, then vulnerable. A well-timed anvil drop can pin it against a wall or ceiling the moment it becomes hittable, letting you finish it with ranged attacks or a strong sword instead of tanking its projectiles head-on.

The nether star it drops feels genuinely rare even though you technically farmed it. It's the only ingredient for a beacon, and beacons are the sole source of several powerful area effects in the game, which is why most players build at least one wither trap even if they never bother repeating it.