Rilian

King Rilian (reign: Narnian-years 2356—unknown) was the son of legendary King Caspian X of Narnia and Ramandu's daughter, fated to become king himself, and born in Narnian-year 2325.

Spoiler warning: Beginning here are "spoiling" details that may relate to the story plot and/or contain plot surprises.

When Rilian was twenty years old, his mother was attacked and killed during a nap in the northern lands by a giant green serpent. From then on he always returned to that place, hoping to avenge his mother's death.

However, Rilian, once up in these northern lands himself disappeared and as many knights as his father Caspian sent looking for him, they all vanished as well. Legend of the Great Serpent in fact being a Witch-queen spread and Eustace Scrubb and Jill Pole were transported into the Narnian realm to rescue the lost Prince Rilian.

When they found the entrance to Underland, a subterranean country, below the Ruined City of the Giants, Eustace and Jill, with their companion, the Marsh-wiggle Puddleglum, were confronted by an arrogant knight who claimed his queen would make him king of a country and they would be married. However, it was said the knight transformed into a Great Serpent each night and so was tied up to an enchanted silver chair to restrain him from being loose in the form of the deadly snake. Upon witnessing the beginning of the transformation, Puddleglum, Eustace, and Jill all realized the knight was in fact under a spell which manipulated him, the enchantment re-conjured each night that he was sat on the silver chair. Just before his transfiguration, the three cut him loose and the knight was turned back to his normal self who was really the lost Prince Rilian all along. The released prince then grabbed his sword and slashed the silver chair to pieces.

Now in a normal state, Rilian and the others encountered the Witch-Queen of Underland herself who at first tried to charm them but then turned into the terrible Great Serpent and attacked Rilian. Eustace and Puddleglum decapitated her and liberated the Underland gnome-slaves who were also under magical control which made them do tireless work without revolting.

The four heroes, including Rilian (apparently 31 years old at the time), made their way up to the surface which was surprisingly on the grounds of the country Narnia where Rilian's old father died within the next few hours leaving Rilian to become Narnia's next ruler.

Although it is known Rilian had a successful regime, certain dates and facts (some from The Last Battle) give reasons that seem to state Rilian's reign lasted exactly "negative one" year, which of course is impossible.

The Black Knight
While he was under the enchantment of the Lady of the Green Kirtle, Rilian's noble and lordly personality was subsumed by one more useful to the witch. This personality, known only as the Black Knight, worshiped the witch, counting her as having saved his life. He was not clever or very intelligent at all, following the witch like an enamored puppy. It was with this personality that the witch would use Rilian to take rulership of Narnia. The enchantment was not perfect, and the Black Knight's personality faded for a brief time every evening. Only the Silver Chair could restore the enchantment, and thus Rilian was tied to it during the time that the enchantment began to fade. Over time, eventually the enchantment would become permanent, and Rilian's personality would be completely submersed under the Black Knight's.