Soul Sand

"Only in the fiery caverns of the Nether, amid columns of lava and flame-belching Ghasts, will you find this creepy material in abundance. At a quick glance it might look like a ruddier version of cobblestone, but then you look a little closer - are those screaming faces etched into its surface? Eep!"

- Marsh Davies

Soul sand is a block only naturally found in the Nether.

Obtaining
Soul sand can be mined with any tool, or without a tool, but shovels are the quickest.

Natural generation
Soul sand is found below Y=65 in the Nether, generally in four-block-deep layers. It is also found naturally in nether wart rooms of nether fortresses.

Usage
Soul sand slows the movement of any mob or player, and causes them to sink 2 pixels into the block while walking on it. Its slowing effect is increased when placed in water, or above ice, packed ice, or slime blocks. It can also suffocate mobs which are smaller than a slab, namely silverfish and endermite.

Growing nether wart
Soul sand is used for growing nether wart, a primary base ingredient for many varieties of potions. Nether wart is planted on soul sand (like seeds on farmland), and at full maturity produces multiple nether wart when harvested.

Withers
4 blocks of soul sand are used in the construction of a wither.

Bubble columns
Placing soul sand under water will create a "bubble column", which will quickly push up items, players and mobs.

Trivia

 * Soul sand can generate on the floor of caverns and lava pools below Y=60 only if it generated in that same column at Y=60 or above.
 * Because soul sand is not as high as a full block, standing near lava while on soul sand will set you on fire.
 * If a water current is pushing you diagonally against a wall and Soul Sand is the block adjacent to your head when the current ends, it will cause you to jump.
 * If snow or carpet is over soul sand, the effects of it being a partial block are avoided, as you will be walking on the cover block, not the soul sand.
 * Due to the ability to reset the Nether in the console edition, soul sand is renewable in that edition. Although you can reset the Nether in the Java Edition by deleting the DIM-1 folder in a Minecraft world file, some may consider this cheating. Also, explored nether fortresses will disappear if you do not delete data/Fortress.dat that stores data of nether fortresses.
 * Walking on soul sand will visibly darken the sky in the Overworld; this effect is less noticeable during the night.
 * This is because of its partial-block nature in combination with the fact that the color of the sky reflects the light level the player is standing in. However, because soul sand is opaque, the player is considered to be in absolute darkness. This may also occur with slabs and other partially-solid blocks that the game considers opaque.