Soul Sand

Soul Sand (A.K.A. Hell Sand or Slow Sand) is a block that was added in Update 0.12.1. It generates naturally in the Nether.

Usage
Soul Sand makes the player and all mobs on it walk slower. The effect is enhanced when placed on Water, Ice or Packed Ice. It can be used to trap hostile mobs.

Soul Sand can also be used to grow Nether Wart, as it can only be grown on the former. However, the Nether Wart doesn't require water to grow.

It cannot be used to spawn the wither boss since its not added yet.

Trivia

 * The player slightly "sinks" while walking on Soul Sand.
 * Silverfish take damage when walking on Soul Sand.
 * This is because they get immersed into the block and suffocate due to their small size.
 * Soul Sand is not a full block, and thus standing next to lava when on Soul Sand will set the player on fire.
 * Soul Sand does not generate in the Overworld, as it is a Nether exclusive block. However players can place it in the overworld.
 * If one looks closely, faces of what could be ghosts are on the block, hence the name 'Soul Sand.'
 * Soul Sand forces all mobs to jump, when they walk on it.