Judge Dryden, from Ultima V: Lazarus
The Avatar, after getting a hint from weapon maker Tactus, met Judge Dryden to learn more about the Oppression. Mentioning Tactus after talking about the Oppression was the key to it. Dryden, pleased with this, sent the Avatar to Flain in Skara Brae, to join the Oppression.
Ultima V: Lazarus Edit
In Ultima V: Lazarus, Dryden had been a clerk and friend of Blackthorn's father during the time he was the high judge. Dryden remained friends and supported Blackthorn after his father's death, and eventually helped him create the Oppression. Dryden was the one that wrote the Ethics of Blackthorn from the guidelines Blackthorn defined.
At Blackthorn's request, Dryden offered the Avatar a deal: if the hero helped the Oppression, they would allow him and his companions to travel freely through Britannia. If the Avatar agreed, Dryden sent the hero to Lady Guinere, who would give the Avatar different quests to help the Oppression. The Avatar could then also pay Dryden the required fines to free Greymarch and Jerone.