Tim

Tim is a Caster skeleton found in the Underground. He can be recognized by his Wizard Hat. Just like any other caster, he will randomly teleport himself to a visible part of the screen after shooting three projectiles. Tim was added as a rare spawn during the 1.0.3 patch. As of the 1.0.4 patch, Tim's spawn rate has been reduced.

Drops

 * : 100% (1:1 chance)
 * Around 5000 copper coins.

Finding Tim
Tim can be found anywhere in the Underground and Caverns layers. Its spawn rate is very low and completely random, but it seems to increase with Depth.

Fighting Tim
Tim is a spellcasting enemy, and shares great similarities between himself and the Fire Imp, and when fighting he should be treated like one. Tim can cast spells that go through blocks, luckly you can deflect his spells with any type of melee object (ranging from Tools to Weapons).

Trivia

 * His name is a reference to Tim the Enchanter from Monty Python and the Holy Grail, this was confirmed by Blue on the Terraria IRC.
 * Tim is basically the Dark Caster with a Wizard Hat.