[OANST]

I wouldn't say that exactly. Most people who have played the game (myself included) consider it to be a prequel to Ico.


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SPOILERS
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At the end of SotC Wander is taken over by Dormin who had been split into 16 pieces and held inside the Colossi to keep him from escaping. Dormin is Nimrod spelled backwards. Nimrod is a biblical character. He was the king that had the tower of Babel built. He was also said to have killed a bull, cut its horns off, and place them on his head, believing this would give him great power. Once the priest Emon threw the sword into the pool in SotC it created a kind of vortex that pulled most Dormin's essence out of Wander. But not all of it. Wander gets sucked into the pool as well. Shortly after, Mono wakes up (which by the way shows that Dormin was honorable enough to keep his side of the bargain). She walks to the pool and finds a baby with horns in it. This is the rebirth of Wander who is still slightly tainted by the essence of Dormin. Later the priest says that maybe one day he can redeem himself.

Now, Ico is definitely not Wander. But he is a decendent of Wander's. Wander was the beginning of the race of horned children that are sacrificed. Ico atones for what Wander has done and loses his horns in the process.