Complex sentence: Slade recruits alderman Sebastian Blood/Brother Blood, who has been running a Mirakuru testing experiment in the Glades, perfecting the serum's effect with Slade promising Blood the mayorship of Starling City in return.
Split into simpler sentences: Slade recruits alderman Sebastian Blood/Brother Blood, who has been running a Mirakuru testing experiment in the Glades, perfecting the serum's effect. Slade promises Blood the mayorship of Starling City in return.