The last time we saw Ronnie Raymond on The Flash was the universe splintering catastrophic event that closed season 1. Ronnie was unfortunately lost as he and his counterpart merged as Firestorm and flew into the black hole in an attempt to save Central City. Doctor Stein, his counterpart, was the only one of the pairing that survived. However this event opened up the multiverse, making it possible to see characters again that have previously died. Previously in season 2 we have seen the return of the alternate version of Harrison Wells. The Earth-2 Wells is actually still alive whereas on the primary earth he was killed by Eobard Thawne and had his identity stolen. In the case of Ronnie Raymond the version from Earth-2 is also alive in the form of an evil version of Firestorm. Amell will return as Deathstorm in the near future. Below is a image of Amell on the set of The Flash as Deathstorm.
The set image was originally posted on Twitter by @canadagraphs.