He was first mentioned in Edern's journal. Not very much was said about Iestin, but it was said that he spends a lot of time in libraries in Prifddinas researching things about elven history. Some of Iestin's research had led him to the conclusion that Lord Iorwerth had somehow broken one of the 9 seals that Seren put in place to protect the Grand Library from the hands of men. He also authored the report from Within the Light, which is found by the adventurer and brought to Arianwyn. The report reveals the fate of Prifddinas, and gives Arianwyn and the adventurer the key information they need to plan what to do next.
During Plague's End the adventurer discovers that Lord Iorwerth has put Iestin to work to invent something to change the crystal seed of Prifddinas to a dark version of the old city. The adventurer then steals Lord Iorwerth's plans from the Mayor of Prifddinas. After reading through the master plan, he becomes shocked and states that Lord Iorwerth was right to keep it away from him, as he would have refused to work on it otherwise. He is then filled in by Arianwyn about the rebels' plan.
Iestin helps the adventurer and the seven other clan leaders to defeat the Dark Lord and activate the Seal of Seren, standing in for the former Lord Iorwerth while the clan leaders attempted to regrow the city. When the city regrew, Iestin became the new Lord Iorwerth, as the next-in-line Morvran relinquished the position to him, knowing that Iestin was moderate enough to represent the Iorwerth after the war.
During The Light Within, Iestin is part of the elven council and eventually Seren is restored during the quest. All clan leaders can be asked for more information about their respective seren shard, but Iestin will say that because of the Iorwerth Clan's past actions, he can not tell much about the Seren shard of integrity and refers the adventurer to Arianwyn. When Baxtorian arrives at Prifddinas to discuss what to do with Seren, he gets into an argument with Iestin due to his Iorwerth past.
In The Voice of the Elders, Volume I, Iestin briefly tells about the history of the Iorwerth and Cadarn Clans, and of relations between the elves and the humans. In it, he mentions that he was born in Iorwerth-occupied Prifddinas, after the Iorwerth Clan had already taken control over the city.
He also explains that, while few elves-even among the Iorwerth Clan-know of the atrocities committed against humans by the Iorwerth Death Guard in West Ardougne, these atrocities committed were so severe that it is quite natural for humankind to blame all of the Iorwerth Clan for their actions.