It includes new content for epic level characters, in the form of extremely powerful, alien monstrosities intent on destroying the world (and designed as a way of providing game masters a means of ending a current campaign).
The book presents nine “elder evils”:
Atropus, the World Born Dead (an undead godling in the form of a small moon)
Father Llymic (an alien monster imprisoned in ice)
The Hulks of Zoretha (five gargantuan statues)
The Leviathan (a monster of the deep composed of leftover chaotic energies from creation)
Pandorym (an evil force from the places between the planes)
Ragnorra, Mother of Monsters (a hideous malformed monstrosity)
Sertrous (a vast demonic snake)
The Worm that Walks (a 30’ tall giant composed of worms and maggots, and connected with the demi-god Kyuss)
Zargon the Returner (a viscous beast crowned with a solitary horn)
Other Elder Evils
A 10th Elder Evil, called Shothragot is presented in Dragon #362. It serves the god Tharizdun.
The D&D book Lords of Madness, published previously (in 2005), also presented Elder Evils (page 27). The five described in that book share a commonality in that they are all greatly respected by the aboleth. They are provided with the following names:
Bolothamogg, Him Who Watches from Beyond the Stars