An instance added by the War Front expansion, the challenge and variety of which is increased by randomization of its content; it changes each time players enter the dungeon. It offers 108 levels of content and otherwise unobtainable or rare rewards.
The number 108 is considered sacred by several Eastern religions, including Hinduism, Buddhism and Jainism, along with their connected yoga and dharma based practices. In Buddhism, it is the number of senses (five senses plus sixth sense) = 6, multiplied by past present and future = 3, negative ambivalent and positive (feelings) = 3, and internal to the body or external = 2.
Perfect World was not the first processor-based game to feature the number; in Sony's Legend of Dragoon for Playstation 1, it was the number of the species the Creator God Soa added to the World Tree, and the number of years between manifestations of the Moon Child.
The number also turns up in a completely different milieu, in Homer's Odyssey: the number of suitors ostensibly courting the hand in marriage of Penelope, wife of Odysseus, which she had to simultaneously entertain, according to Greek customs of hospitality, and rebuff, as she waited faithfully for Odysseus to return to Ithaca from the siege on Troy. The suitors were actually exploiting the customs of hospitality for food and wine, and when Odysseus finally made it back to Ithaca, he avenged Penelope and himself on them in a battle that was bloody but which nonetheless displayed his characteristic cleverness with its tactical innovation. It seems likely this is not mere coincidence and that the Greeks had their own version of the mythical/mystic appelation to the numerical "palindrome" 3 x 2 x 3 x 2 x 3