Complex sentence: As Holmes and Blackwood brawl, Holmes unmasks Blackwood's supposed "magic" as nothing more than mere conjuring tricks, before tricking him into becoming entangled in the ropes and chains hanging over the Thames.
Split into simpler sentences: As Holmes and Blackwood brawl, Holmes unmasks Blackwood's supposed "magic" as nothing more than mere conjuring tricks. During the fight, Blackwood becomes entangled in the ropes and chains hanging over the Thames.