Probe his motives in offering himself as substitute
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From: Conversation with Cornelius or Alone with Cornelius
Why would he even consider standing in Furnace's place?
Unlocked with 
An Eager Substitute in the Form of Cornelius exactly 1
Success
A non-answer answer
[...] he produces from his pocket a large brown rat in a brass helmet,[...]
"It doesn't have three mouths," Cornelius admits regretfully. "[...] But the helmet is natural, [...]
[...]the helmet appears to have grown from the rat's flesh.
[...]
An occurrence! Your 'An Eager Substitute in the Form of Cornelius' Quality is now 2!
