It's on Diana the Valkyrie's site.
Diana's amazon growth stories in general:
http://www.thevalkyrie.com/stories/growth.htm
The story you're looking for; The Spell:
Part 1:
http://www.thevalkyrie.com/stories/marknew/spell1.txt
Part 2:
http://www.thevalkyrie.com/stories/marknew/spell2.txt
Part 3:
http://www.thevalkyrie.com/stories/marknew/spell3.txt
I know it is in other places complete, not in parts, but I don't remember them.
Enjoy Jamball!