Arbiter

Arbiters are hand-shaped monstrosities which essentially act as an enhanced version of Inquisitors. These enemies are only found in the Cavern.

Behavior
Arbiters, like Inquisitors, only have one attack. They can spot players through walls and will attack them from afar, sometimes even off-screen, though they can attack only horizontally. Their attack consists of a flurry of 5 magic bolts distributed to cover an angle of ~90° in the direction they face, with the central bolt directly aiming for the player.

Upon death, Arbiters drop a grenade which fires projectiles in a hexagonal pattern. It behaves like a normal grenade if struck by an attack that returns projectiles (shields, Wave of Denial, etc). The spawned projectiles can also be parried.

Unlike most mobs, Arbiters cannot teleport to the player on Nightmare and Hell difficulty modes, and only one arbiter may attack the player at any given moment if multiple ones are present.

Strategy
To avoid getting hit, it is advised to bait them individually into attacking and parrying their attacks. Despite firing multiple projectiles, only one of them can effectively hit the player at range, making them similar to Inquisitors in a sense.

When wielding an off-color shield, however, parrying is less effective. Another safe option is to use the Homunculus rune to slowly kill them, as the range of this rune is larger than the Arbiter's detection field. In situations where neither of these approaches are possible (e.g. no shield, cursed), one can try to bait them on one side of the screen and quickly kill them from behind while they are firing at their previous location.

Elite Arbiters are ironically often easier to deal with than normal ones since they teleport and can be lured away. A notable exception to this is when an Elite Arbiter spawns with the electric cage ability, which makes it particularly hard to kill without getting damaged.

Trivia

 * Arbiters were added with the v1.2 update, aka Rise of the Giant DLC in March 2019.
 * Before v1.3, the Update the 13th, Arbiters were common enemies in most biomes at 3+ BSC. However, this resulted in excessive projectile spam and thus they removed and replaced with Rampagers.