"I am he, Judge Sleen, Master Sage and Arcane Historian."
—Sleen, to Xeron
Might and Magic VII
Both Queen Catherine of Erathia and King Eldrich Parson of AvLee lay claim to the Contested Lands. To prevent a war, an arbiter was appointed to settle the matter. When Arbiter Grey died of a heart attack, they asked the Lords of Harmondale to find a new one.
Gavin Magnus advised them to pick Judge Fairweather, a representative of the wizards of Bracada and a charismatic man who made fair and just resolutions. Archibald Ironfist, on the other hand, wanted them to pick Judge Sleen, a representative of the necromancers of Deyja and known for his habit of not getting involved in disputes he was asked to arbitrate.
If the party picks Fairweather, Erathia and AvLee find a peaceful resolution. If the party picks Sleen, war breaks out, and the necromancers of Deyja raise the corpses to create an undead army. This choice decides whether the party follows the good path or the evil path through the rest of the game. Canonically, Fairweather was the arbiter.
When Xeron was trying to create Armageddon's Blade, he asked his servant Xex to locate a knowledgeable sage. Xex returned with Judge Sleen, and Xeron threatened to eat the man's heart unless he told him everything he knew about the Conflux heroes that Xeron was fighting. Once Xeron had the information, he asked Xex to teleport Sleen away.
Sleen returned a week later with more information he'd discovered. When he'd given his report, Xeron asked Xex to transport him somewhere "unpleasant". Xex took Sleen to the middle of a battle between Dracon and some faerie dragons, as this was "as unpleasant as he could get without killing him."
Throughout the war, Xeron would ask Xex to fetch Sleen whenever Xeron needed new information. Sleen was terrified that Xeron would carry out his threat, so he eagerly told Xeron everything he knew.
As Gelu's forces drew near Kreelah, they managed to capture Sleen. After hours of questioning, he told them about Xeron's plans. Sleen was sent to Steadwick in chains to stand trial for his crimes. General Kendal appointed Johannes Bonn to be his judicator, and Johannes sentenced Sleen to spend the rest of his life in prison, where he would be doing hard labor in solitary confinement "so he may contemplate that which he has done."