Initially a stretch goal for the Ember: From the Ashes Kickstarter, it did not get unlocked. It was then offered later to backers.
From the email offer:
"This 10 card expansion allows you to capture the Purple Worm, a fearsome creature that can reap havoc across the Ember playing field. Gain control of the Worm to capture creatures and swallow them whole, then use your body segments to protect the head from destruction.
One thing is certain: when you bring the Purple Worm into play, battle us inevitable. For advanced Ember play only!"
NOTE: There was an error in the Purple Worm offer saying it consisted of 1 Head, 1 Tail, and 8 body segments. It was always intended to work with two heads and tails. Backers were given the choice of receiving the Purple Worm expansion in one of two ways:
Option A: 1 x Head, 1 x Tail, 8 x Body Segments
Option B: 2 x Heads, 2 x Tails, 6 x Body Segments
So both versions will be out there.