The Prince of Thieves was a real devil when he first appeared late in Season 2, played by Welsh actor Tom Ellis of Lucifer fame.

When the fantasy drama targeted Robin for a long-term run, Ellis was busy headlining USA Network’s Rush, and English actor Sean Maguire was cast.

https://tvline.com/lists/recast-tv-roles-explained/once-upon-a-time-robin-hood/