Sulphur

Sulphur is an item dropped by Warped terrorbirds and Warped tortoises. It can also be picked up to the north of the Poison Waste. It can be ground using a pestle and mortar to make Ground sulphur, which is used in Regicide to make the Barrel bomb.

Trivia

 * Sulphur was made untradeable in the 20 November 2007 update because some scammers used to sell it for a high price, claiming it was rare.