POW Blocks are red or blue blocks with a white POW on them. They appeared in games such as Mario Bros. and Super Mario Bros. 2.
In MKFusion, these POW Blocks are of the SMB2 design. They can be picked up and thrown onto the ground, defeating all enemies on screen. POW Blocks are useful against enemies such as Covenant Elites or groups of Hammer Bros., where they kill instantly. POW Blocks are really common in The Library, where you need these to take out swarms of Carrier and Combat Forms, despite being a Halo-themed level.