Silverfish

Silverfish are small, bug-like hostile mobs which have silver skin and black eyes that hide in monster eggs found in strongholds and extreme hills. The blocks look identical to normal blocks, but when the block is mined, a silverfish pops out and attacks.

When attacked, silverfish may awaken other silverfish within a 21x11x21 area. This can lead to a terrifying swarm if there are enough in the vicinity, and an uncontrolled swarm can quickly surround and kill the player. Left alone, silverfish will re-enter in another block, transforming it into another monster egg.

Spawning
Silverfish don't spawn naturally, but spawn from monster eggs and monster spawners, which are only found in strongholds and randomly underground in extreme hills.

When spawning from monster spawners, they can only spawn in light level 11 or lower, except on top of stone, where they can spawn at any light level. They also cannot spawn within a 5 block distance of any player. These restrictions only apply when spawning from a monster spawner, and not from a Silverfish block.

Combat
The safest way to engage silverfish is by standing on top of a 2-block tall pillar, which will prevent the silverfish from reaching the player. The player then can then kill the provoked silverfish by using non-direct attacks, such as dumping a bucket of lava or dropping gravel. Non-direct attacks will not awaken other silverfish. Using splash potions of poison can be considered the worst way of combating silverfish, as each damage tick they take from the poison, it'll alert other silverfish in the area, immediately producing a large swarm.

If a silverfish is killed in one hit, it will not alert other silverfish, so a diamond sword is recommended to combat silverfish. Bear in mind that when silverfish re-enters a block, they are cured of any effects and restore any lost health.

Farming
Some people build Silverfish XP Farms for enchanting and anvils. They use the end portal's silverfish spawner for spawning silverfish.

Here's a tutorial on how to make one:

Trivia

 * Silverfish are the smallest mob in Minecraft.
 * In real life, silverfish are small, wingless arthropods that are often found in dark areas in buildings and the wooden pilings of seaside piers and trees. They are infamous for damaging paper-based products.
 * Real silverfish are very small, no larger than a few inches. During cold temperatures they are known to enter a sort of "hibernation" by expelling most of the water in their body. During this state they greatly resemble small stones or gravel, likely inspiring their Minecraft counterparts.
 * Silverfish can be killed with fire, lava or explosions without the Silverfish turning hostile, preventing them from calling in more allies, so long as the player does not hit them directly.
 * Silverfish are often used in servers as a form of griefing. Greifers will place monster eggs in players' structures, and unknowing players would get attacked by silverfish as soon as they break it. If the player is inexperienced and attack the silverfish on sight, this would result in more silverfish spawning, which will cause the death or stranding of the player.
 * When the difficulty is set at peaceful, breaking a monster egg will spawn a silverfish, but the silverfish would de-spawn instantly.
 * Silverfish will never stop moving unless forced into a one-block hole.