Okay, I'm pretty sure that 50% of people who answer this thread won't read more than the title, so I expect mostly "Who is Erasmus?" and "He's the iconic for mediums, guess what he is!" answers.
Now the ugly truth is, that Erasmus and the Medium have very little in common, fluffwise and ruleswise. Let's start with fluff, because in the end it doesn't matter, everyone can change it around to fit what they like. The description of the Medium says: "A medium serves as a vessel to channel spirits—astral echoes of powerful souls from ages past that live on in myth and legend." Well, I guess it's pure accident that the "spirits of legend" the medium can channel all share names with mythic paths. If Erasmus' uncle, sister and brother were legendary, mythic beings he's kind of the underachiever of the family. And his brother a genius for killing them all. But as I said, that doesn't matter, a medium that channels the spirits of dead relatives is an awesome idea, so now we come to the ugly part, let's look at the story of Erasmus:
"With the aid of his uncle's mastery of magic, Erasmus easily retrieved the keys to his cell to secure his escape. Under the control of his battle-eager eldest sister, the asylum's guards had no hope of restraining him. But when he gratefully burst open the King's cell, he found it empty. Too harried to ponder the implications of his friend's disappearance, Erasmus fled, relying on the influence of his roguish brother Baylock to slip through the shadows to freedom."
Sounds awesome, doesn't it? I really want to be like that guy.... So let's play through that scene with the actual medium rules:
"Erasmus spends one hour channeling the spirit of his archmage uncle (because a cell in an asylum is obviously the right location to channel an archmage), then he uses his magic powers to retrieve the key to his cell. He spends the rest of the day waiting until he can perform another seance. Then he spends one hour channeling the spirit of his champion sister (again his cell seems to have turned into an "appropriate location" for the spirit he needs) and breaks out, overwhelming the guards who were trying to restrain him. He also tries to break out the cell neighbour who taught him about being a medium but he's gone. So now Erasmus waits for the remaining twentyfour hours (I don't get it, did he murder all the guards? Wouldn't they wake up otherwise? Then why does he need to hide?) and until he can spend one hour to channel his trickster brother Baylock (he obviously found an "appropriate location" in the asylum for that) "to slip through the shadows to freedom".
Seriously. What sounds like an evening's work in the background story takes at least two days with the rules. We really need an archetype for someone who can channel blood relatives without silly twentyfour hour limits location restrictions.