This a Homebrew Tap containing a collection of formulas/casks that have not yet been accepted into the mainline Homebrew/Caskroom repo.
First, add this tap to Homebrew:
brew tap thegiantbeast/tap
Then you can install these formulas/cask as if there were in canonical Homebrew/Caskroom repository:
brew cask install expressvpn
This software is distributed under MIT license.