EverQuest: The Serpent's Spine is the 12th expansion to EverQuest, released September 19, 2006. New features include new zones, a new player race known as the "Drakkin", an attainable player level of 75, as well as new spells and alternate advancements.
The expansion was added with the aims of making soloing (playing Everquest alone rather than as part of a larger group) a more viable option. This was done through the addition of a new town, home of the newly added Drakkin race, which provides various tasks for all levels of game play to gain experience, gear, and money. The advertisements for "The Serpent’s Spine" stated that one could go from level one to level 75 without a group or exploring any of the previously existing locations in the game simply through completion of these newly added tasks.
The Serpent's Spine expansion also added the rest feature, allowing players who have been out of combat for a short period of time to regenerate mana and health at a drastically increased rate. This effect was added to aid those soloing, as well as to solve complaints about down time or med time spent meditating to regenerate lost mana. While generally greeted well by the Everquest community there have been complaints about the new rest effect causing class balancing issues. This is due to the ability for some classes to expend their mana rapidly to cause large amounts of damage only to regenerate it with the new rest effect in the time it takes for their group to kill a single monster, thus increasing the total damage done in a group compared to the previous method of conserving mana to ensure a steady supply. This causes classes that are unable to utilize this method to decrease in their comparative effectiveness.
Source: Wikipedia, "EverQuest: The Serpent's Spine", available under the CC-BY-SA License.