Blacksmiths who have a character level of at least 50 and a skill level of at least 265 could learn how to create pieces of the Imperial Plate armor set. In order to get the plans for each piece, you needed to turn in Thorium Bars to Derotain.
As of Patch 4.0.3, all of the Imperial Plate set are trainable directly from Blacksmithing trainers and Derotain Mudsipper no longer offers these quests. This NPC therefore has no in-game function, although he remains in his old location
Trivia
The disdainful attitude he gives players comes from the expected, and generally accurate, reaction to requiring such large amounts of Thorium for the plans. Though recent patches, not to mention two expansions, have expanded available Thorium to players, these plans are still amongst the most expensive to acquire and create. At one point, before a patch reducing the cost, acquiring and crafting the set required 384 Thorium Bars. For this reason, being able to craft the entire Imperial Set was once considered an impressive feat undertaken only by the truly dedicated or insane.
Patch changes
Patch 4.0.3 (15-Nov-2010): No longer a quest giver.