Haven was the Age Achenar was imprisoned for 20 years.
Atrus wrote Haven as a Prison Age in order to protect his library on Myst. Taking a lesson from Stoneship, he wrote a ship into the Age in order to make Haven seem an inhabited world, and attract greedy explorers who would long for riches and adventure.
He made one sole foray to make sure that the Age can support human life. He saw several of the wild beasts from the rainforest. While walking under the nests of the indigenous Mangrees he was dismayed that he did not have the time to stay longer and let them become his friends. He then took few brief minutes to watch the sun set over the freshwater lake, then swam out to the middle and linked back to Myst, letting the linking book fall in the swamp and be destroyed. That evening he wrote two linking books, one for Haven, and one for Spire, and placed them on display in the library. He warned Catherine, Sirrus and Achenar to stay away from them.
Neglecting his father's advice, Achenar linked to Haven, lured by the wrecked ship. Realizing that he was trapped, he built a base and became obsessed with hunting.
The shock of finally realizing that he had killed innocents drove him completely mad. The mangrees which had socialized Achenar as one of them played an important role in his final repentance.
The "good" influence of Haven on Achenar can't be compared to the influence that Spire had on his brother Sirrus.
Haven was mainly a jungle full of wild animals. There was also a beach on which there was an old ship merged into the causeway near Haven's coastline, which Atrus purposefully wrote into the Age in order to lure greedy explorers. Animals that lived on the beach were karnaks and cerpatees, the latter of which was rendered extinct. Near the beach, Atrus wrote a linking chamber which they used to talk with their son without the danger of his escape.
Achenar used a base made of jungle materials to survive and became obsessed with hunting.