Watcher

The Watcher is a hostile, post-Plantera mob found in any of the Infection biomes(Corruption, Crimson, etc.). 3-5(4-7 in Expert Mode) are found following a Greater Watcher. This is the only way they spawn naturally.

Stats and AI
HP: 1000

Defence: 30

Attack: 50 (Melee), 300 (Last Stand)

KB Resist: 20%(35% Expert)

The AI of the Watcher is inbetween that of a Drippler and a Creeper, with one exception. Normally, it flies around randomly, but if it gets too far (30 blocks) away from the Greater Watcher it rushes back to it at a very fast rate, dealing double damage to anything that happens to be in it's way.

Once it is killed, the Greater Watcher it was following is killed, or it leaves the Infection biome, it starts spinning rapidly for about five seconds before charging the player with an attack know as "Last Stand". In Last Stand it becomes an invincible projectile that moves at a rate comparable to the max speed of a Unicorn mount towards the player. If it misses then it flies off until it goes 50 blocks past off-screen when it despawns, but if it hits it deals a ton of damage. Get your dodge game on, or you're in for a lot of trouble!