Hello there! My name is John and I am writing a story. I based the story very much upon a certain term, a certain word. Only the problem is; I’ve forgotten the word!
It’s kind of strange, yes, I know that, however, I’ve taken some awful hits to the head in my lifetime. Therefore I sometimes get these forgetful phases, or phases where I can’t remember something very specific, in this case, that blasted word!
When I try to remember the word, all I can think is, armor or conduit.
From my notes I know that the word means something like, “the thing which takes the strain”.
In my universe there are basically two kinds of sorcerers. True-bloods, who can use magic at will, with no permanent cost to themselves. And then everyone else.
And that’s where you guys come in! 
Specifically: If you are a normal person, who knows magic, you cannot use it at will, because then it would wear you down, age you or otherwise harm you, that’s why you need a “X” because it will take the strain of using magic.
