Fantasy Novel by Ursula K. Leguin, the third novel in her Earthsea Trilogy1. * * * * * (explanation)
Something is very wrong in the Archipelago of Earthsea.
People everywhere are leaving their responsibilities and taking up a charismatic cult that promises Eternal life No, not Everything2.
The Archmage of Roke, Sparrowhawk2, suspects he knows what is wrong. Embarking on a journey with Arren, son of one of Earthsea's great lords, he decides that part of the problem is that Earthsea has not had a king for 3000 years. However, he suspects the problem lies deeper. Someone has opened a door that should not have been opened. Sparrowhawk knows this because he opened it once himself.
1The other novels are called A Wizard of Earthsea and The Tombs of Atuan.
Never reveal your true name
to anyone but your closest, most intimate friends, for it gives them power over you.