Soul Sand

Soul Sand (AKA Hell Sand or Slow Sand) is a block that will be added in Update 0.12.0. 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 thus 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.

Trivia

 * Silverfish take damage when walking on Soul Sand.
 * Soul Sand does not generate in the Overworld.