Pack Form

The Pack Form is a druid shape shift ability that allows the druid to take the form of a wolf. This form is known for its power and ferocity that embodied the fury of the wolf ancient Goldrinn. Unfortunately, those who undergo this form tend to lose their minds to the form's feral animal instincts. It was first discovered by Malfurion Stormrage, and during his time spent consumed by the form's rage he blindly attacked Cenarius. Due to its uncontrollable nature, Malfurion prohibited the use of this form during the War of the Satyr.

Druids that disobeyed Malfurion's edict became known as the Druids of the Pack. Their efforts to control and master this untameable form led to the creation of the worgen. Despite their intentions, the Worgen still suffered from the Mindless state that the form put them in, which resulted in Malfurion's banishing the druids to sleep eternally within Daral'nir in the Emerald Dream, mourning the loss of his fellow druids. Malfurion Stormrage was the first to discover the pack form but was unable to control it and even attacked his shan'do, Cenarius. He regained control of his mind when his rage was soothed in Daral'nir, the Emerald Dream counterpart to Tal'doren. This experience with the pack form is the reason why he forbids the use of the pack form

Information taken from:

http://wow.gamepedia.com/Pack_Form