Mining Fatigue

"During the development of the ocean monument I had a problem. I wanted the pyramid to feel like a maze, so you have to figure it out to get the treasure, but the bad – and good! – thing about Minecraft is that you can just rebuild everything. So the solution would always be just to knock your way through. And I didn't want to make the pyramid indestructible because that's also not fun in Minecraft. So after a couple of ideas we decided to have a boss guardian that would have a spell effect that would slow everything down."

- Jens Bergensten

Mining Fatigue reduces mining and attack speed, opposite to the Haste effect.

Effect
The effect decreases speed by 10% × level. Mining speed decreases more than movement speed. The effect lasts for 5 minutes.

Negative levels increase attack speed, but still act as level 4 with respect to mining.

While afflicted, a player can still craft and ride entities as normal, but cannot remove most vehicles due to how slowly the vehicle can be broken.

The affliction is cured by drinking milk, although milk does not prevent a subsequent affliction by an elder guardian.

Causes
The only natural cause of Mining Fatigue is from affliction by an elder guardian, and the only natural effect level is III. Once per minute, elder guardians in the vicinity search for any player within 50 blocks to afflict with Mining Fatigue III. A player cannot defend from this attack. Concealment behind blocks, underground, or by a potion of Invisibility offers no defense.

Other levels can be achieved via commands.