Freedbot
Terrarian
No fancy gif or notation for this one. It's as simple as it gets. I suggest using TEdit for this one, but the teleporter floating trick may work as well. I haven't tested.
How it works: Before hardmode, only some basic grass blocks and vegetation can spread corruption to other similar vegetation. When a world transitions to Hardmode the list is much bigger. When any of those stone blocks gets corrupted, it sends a block update to the boulder, which reminds it that it's supposed to fall when no blocks are under it. The boulder counts as a hostile NPC to the pressure plate which sends a single pulse. The background wall keeps mobs from entering. A layer of lava works too. Obviously this works just as well with ton's of other block combinations and other purposes: ice, mud, grass, chlorophyte, anything that spreads, converts, or grows can cause a block update. The timeframe to trigger is somewhere between "instant" and ~1.5 minutes. To reduce the potential delay, add more corruption blocks in range or chain multiple of these together to a self-disabling faulty gate.
For using this in a custom map it's best to hide it somewhere the user won't see and where the hardmode strips won't hit it. I THINK those stripes might not cause the necessary block update. Personally, I needed this to destroy some temporary biomes I had placed outside the world border in my map while updating it for summoning the Deerclops.
Brutal Skyblock On Curseforge
...On Steam
...and more of the wiring from it posted here.
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