https://wccftech.com/todays-list-of-paid-apps-and-games-that-are-free-at-the-play-store-including-undead-tower-crusade-and-more/

The titles are on free for a limited time only and may go paid at any moment. If you see a price next to the “install” button on the Play Store, it isn’t free anymore. Once you download a paid title that’s free, it’s yours to keep even after the promotion ends and you can reinstall it on any other device linked with the same Google Play Store ID.

Related Today’s List of Paid Apps and Games That Are Free at the Play Store Including Pirate Defender Premium and More