He's here to make casting spells a nightmare - Loatheb increases the cost of all enemy spells by 5 for the next turn.
Curse of Naxxramas spoilers have been silent for a while after the reveal of the last class card - Hunter's Webspinner. At that point, there was no information whether or not Blizzard will reveal more cards before the Curse of Naxxramas launch (announced yesterday to be on July 22nd) but apparently the answer is positive.
A 5/5 for 5, Loatheb's Battlecry increase the cost of all enemy spells by 5 for the next turn only. This effect looks to be incredibly strong when applied in the right moment and can be used to stop Miracle Rogue's drawing power, Shaman's and Mage's spell burst or lethal combos like Druid's Force of Nature/Savage Roar. Loatheb can not only save his owner from lethal but also indirectly protect allied minions by putting weights on opponents' removals - a 6 mana Execute or an 9 mana Swipe are certainly no good deals.
