Tim

Tim is a Caster Skeleton enemy found in the Cavern layer. Appearing similar to Dark Casters, he can be recognized 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.

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.
 * Tim seems to spawn more commonly the closer you are to The Underworld.

History
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