You must have the free-to-play base game first. Just click Play Game and it'll be added to your library. Then you can get the Accursed Army Pack for free. To keep them forever you must claim before May 31st.
Minion Masters is rated Very Positive from 15,357 reviews. Both game and DLC include Steam trading cards.
Also available on the Microsoft Store.
Best get this from Microsoft. Add to library and install later.
Problem with Steam is you have to install the free to play base game to add the free DLC the content. Doesn't seem to be a way to add free to play games or DLC content to your library and install them later on.