The Vilethorn is an early-game magic weapon obtained by breaking Shadow Orbs in the Corruption. It fires a thorny spear that can go through walls and lingers for a bit. As such, it is an amazing PvP weapon before players get their hands on a shield of some sort, despite its underwhelming damage.
The Vilethorn's ability to linger for a while is an excellent stunlocker - it keeps players away for all the time that it's there. In addition, multiple thorns fired at once (it is possible) will deal more damage and stunlock them permanently. However, shield-toting players will find stunlocking a breeze to charge past.
The Vilethorn's ability to attack through walls also makes it extraordinarily deadly in traps or just camping. Protected by a shield of blocks, one can rip away at an attacker. Or, hidden in a dark hole beneath the earth, one can send thorns up at players running over. I've had both these strategies used against me, and they both really work.
Good armor and a shield are the best defense against these fairly weak, stunlocking hits. Also, make sure to avoid dark places where campers could hide, and tote a good pickaxe so you can break through the defenses of Vilethorn campers. In addition, a Vilethorn of your own allows you to get at them.
Keep in mind that, until you get close, you have nothing to fear, as the Vilethorn has a fairly small range compared to other early-game magic weapons.
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