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. Soul sand slows down the movement of mobs and players standing on it. Soul sand can be used for growing nether wart too. It also creates blue fire when you use flint and steel on it. SPOOOOKY.

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. And it can be ALSO obtained in the soul sand valley. In soul sand valley you easily find soul sand since its covers every wall, roof, and floor.

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, endermites.. And baby turtles, who would do such a thing :(.

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. This also unlocks an achievement "the beginning". More info in Gametheory/Mat Pats channel.

Bubble columns
Placing soul sand under source water blocks creates a lifting bubble column, which causes items, players and mobs inside the column to rise to the surface. The bubble column extends upward through any number of empty water source blocks (not waterlogged blocks).

Note blocks
Soul sand can be placed under note blocks to produce cow bell sounds. diffrent from the villager raid bell.

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 the player on fire.
 * If a water current is pushing the player diagonally against a wall and soul sand is the block adjacent to the player's head when the current ends, it will cause the player to jump.
 * If snow or carpet is over soul sand, the effects of it being a partial block are avoided, as player will be walking on the cover block, not the soul sand.
 * Due to the ability to reset the Nether in the Legacy Console Edition, soul sand is renewable in that edition. Although a player can reset the Nether in 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 the player does not delete data/Fortress.dat that stores data of nether fortresses.
 * When heads are broken, the resulting particles have the soul sand texture.