Ignoring the fact that it's not a full sentence, I think it should be grammatically correct because you could say


  [This is] Where evil happens


so it's just a different adjective, but using amazing there just sounds ugly/wrong to me. 


  Where amazing things happen


sounds much better to me.

"Where Amazing Happens", is this poster slogan a grammatically correct expression?
why when i comment \usepackage{geometry} i get error?

\documentclass{book}

\usepackage{geometry}

\makeatletter
\@ifpackageloaded{geometry}
 {                               % i the package was loaded 
    \newcommand*{\qw}{\itshape}  % this is enabled, else is not enabled.                                                                                               
 }{}
\makeatother

\begin{document}
\qw hallo
\end{document}

