From the introduction:
Long ago a sect of elves made a pact with the devil Gheryon. In exchange for winged flight the elves gave away their long life. The aerial elves crafted flying fortress and waged war on their kin. The surface elves united and used their combined magic to send the fortresses crashing to ground. The surface elves thought the aerial elves were extinct but one fortress and its lost race of winged elves survived. Now an Ogre Mage Jhang has found the fortress and is using it to plunder the kingdom. He is aided by hippogriff riding orcs and the cruel aerial elves. The elf lord Faeradoon has quested you to fight the evil and be rewarded richly. His direct assault failed and his forces never returned. He bids you use stealth on this attempt.