This song is written by Dan Reynolds, Daniel Platzman & Wayne Sermon and produced by Imagine Dragons and Mattman & Robin.
"Believer" is in A sharp Minor key, for piano player you must transpose up 1/2 tone to play this song with simple version. It use just 4 chords with IV V IIIM and VI IV IIIM progressions and use E as substitution chord. Use Down Down UpDownUpDown Down strumming pattern per 2 chords and you need to change chord just before the third beat.
See example of chords lyrics here:
But they never did, ever lived, ebbing and flowing
Till it broke open and it rained down
It rained down, like...