I used to be part of a sci/fantasy book club and I remember getting a compilation of the years best sci/fi-fantasy.
One of the stories revolved around a young man who worked in a toy shop? and he would be “summoned” to kill people for revenge. When he was summoned he glowed green? I remember the main focus of the story was your personal magic was based upon how long your mom was in labor with you. And the magicians kept his mom in labor for months in order to make him practically invincible.
I really enjoyed the story and would really like to read it again. It was over thirty years ago, so hopefully someone can help. Thanks.