Help me Open the Box
JavaScript ES6, 57 bytes

s=&gt;s[r="replace"](/-+/,s=&gt;s[r](/-/g,`
$'/`))[r](/-/g,' ')


Outputs a leading newline. Works by taking the row of -s and converting them into a triangle, then replacing the -s with spaces.

Edit: Saved 5 bytes thanks to @edc65.