This mod adds just one block: the Mining Fatigue Emitter. As the name suggests, it "emits" the Mining Fatigue effect to an area around it to discourage dirty cheaters from cheesing your puzzle rooms in survival!
The block itself is immune to explosions and has a similar mining time to obsidian, making it hard (but not impossible) for it to be broken by a survival player. However, there's no way to craft it and it doesn't drop anything when broken. You can't push or pull it with pistons, either.
However, the block doesn't do anything on its own - it only activates once redstone power is either supplied or removed. When activated, it inflicts Mining Fatigue 4 in a 25x25x25 cube centered on itself. Hooking this block up to a redstone clock will allow the block to apply the effect at the interval you want, meaning if someone drinks milk to counter it, you can choose how much mining they can sneak in (if any). Since it only activates when its redstone supply changes, this also means that the emitter can be disabled by constantly powering it. One way this could be used is setting up some sort of puzzle that can disable the emitter's effects once solved, allowing players to mine in that area (provided you didn't overlap more than one emitter, that is).
This mod was made with the intent to be used with others mods that allow players to add their own structures, or for making custom maps where you want to discourage mining in specific areas.