The Character of Arcadius in the Riyira Series

So, when I'm first introduced to Arcadius - keep in mind, I've listened to The Riyria Chronicles first and haven't started the Riyria Revelations yet.  Though I have finished Legends of the First Empire with Graphic Audio.  

So, I do what I usually do when I come across a seemingly mysterious character: I got to Google to look for some crumbs of information. And I got a bit of a blurb that I posted below.  

Let me just say, my first impression of him was rather avuncular or fatherly, with some hints of shady secretiveness thrown in. And I like him, but there's that sly underhandedness that has me wanting to look harder.  Like lying about Pickles/Rhen's fate to Hadrian.  It's the first clue that he's more a spider in a web of his own making but I want to give him the benefit of the doubt.

For now take a look at what Google's told me below and later, I may add more:

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 Arcadius in the Riyria Series is a pivotal, complex figure—a wizard and former family friend of Hadrian Blackwater who becomes a mentor and guardian to Royce Melborn, orchestrating events to protect the hidden heir of Novron (Royce) from the church, though his well-intentioned, elaborate plans often backfire, making him an accidental antagonist by inadvertently leading to tragedy, like Gwen's death, and driving the heroes apart in his misguided quest to guide humanity's future. 

Key Aspects of Arcadius:

• Mentor to Royce: He took in the young assassin, Royce, after his family's demise, providing him education and a home in Sheridan, working off a debt.
• Guardian of the Heir: He sought to protect the true heir to the throne (Royce) from the corrupt Church of Nyphron, believing it essential for humanity's survival against the elves.
• Master Planner (and Schemer): Arcadius orchestrates major events, hiring people like Rehn Purim to guide Hadrian, and using his own magic to find the heir, but his complex schemes often have unintended, devastating consequences.
• Source of Conflict: While trying to do good, his actions, such as revealing the heir's location or orchestrating events around Mercy, directly lead to heartache and alienate Riyria.
• Identity & Motivations: He's driven by deep concern for the world's fate, but his methods are manipulative and flawed, making him a tragic figure who clashes with the protagonists. 

In essence, Arcadius is the enigmatic wizard who sets the stage for much of the Riyria saga, a benevolent but meddling force whose grand vision for the world inadvertently creates the very conflicts and tragedies that challenge Hadrian and Royce. 

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What incident caused him to give up on Royce and focus on Mercy instead?