I was looking for the book Amphisbaenas Book Of Infinity and can't seem to find the battle challenger "Amphisbaena". I've found the Stone Amphisbaena but that dropped a ring. Where was the Amphisbaena found?

Stone Amphisbaena was a Blake Steele Plot Challenger back in 2011 (more info).

As for the book, the Stone challengers of the plot were rereleased for a short time last year and it appears that was the time to get it. It's retired now.

Oh, pity. Thank you, .

there has been no official word, however, that the Stone challengers will never come back. In fact, there's even the chance that they will be the next Throwback challengers that should rotate today. Anyway, keep on waiting. They will eventually come back and might have their previous loot back with them.

That would be great!