Tim

Tim is a Caster Skeleton enemy found in the Cavern layer. With an appearance similar to Dark Casters, he can be easily distinguished by his Wizard Hat, which is guaranteed to drop when he is defeated. Like any other caster, Tim will shoot three Chaos Balls that pass through solid blocks before teleporting to another visible location on screen. Tim is immune to the Poisoned debuff.

On the, wearing one of the gemmed variations of Robes will increase Tim's spawn rate from 1/200 to 1/50.

It is one of the few enemies that can be detected by the Lifeform Analyzer.

Achievement
There are Some Who Call Him... is the achievement you earn for defeating Tim for the first time.

Trivia

 * Tim's spawn rate increase is described in changelogs as occurring if the player "need[s] his hat". This bonus is actually triggered by wearing one of the gem-crafted variations of robes- all of which can serve as half of a two-piece armor set with the Wizard Hat that Tim drops, granting the player a 10% increase in magic critical strike chance.
 * Tim's name is a reference to Tim the Enchanter from Monty Python and the Holy Grail, as confirmed by Blue on the Terraria IRC. The name of the achievement gotten by killing him is also a reference to his introduction in the movie.
 * Tim seems to spawn more commonly the closer you are to The Underworld, though he will not spawn in The Underworld itself.

History
Tim Tim Tim Тим Tim