Well, I hate to say it, but you are out of luck sadly, as that quote from Fade to White was really all that was revealed on the subject. The Path of the Twelve storyline in 1.0 was sadly abandoned without being completed when Yoshi took over as producer as the storyline was repurposed for the Seventh Umbral Era.

I've heard that datamining of the 1.0 client however has apparently revealed dialogue glimpses of how the Path story was to continue after it's final completed mission Future's Perfect, where the player is sent to investigate the kobolds only to inadvertently stumble into Titan's very first summoning (and the resulting battle between Titan and the Company of Heroes), but nothing is known with certainty (and there was no further discussion about the 'shameful truth found at the end of the Path' either).

I could speculate of what it was exactly the 'shameful' truth that members of the Path of the Twelve inadvertently uncovered, but as I'm usually wrong when it comes to speculating on how the storyline turns out I'm going to keep silent for now.