When the two strongest heroes in the Marvel Universe, Sentry, fused into one being, fans expected them to become one of the most powerful and epic characters that the House of Ideas has ever produced. Instead, this short-lived fusion fell very short of expectations and managed to do absolutely nothing relevant for the whole time he was around. However, no idea is ever lost in comics, and there's still time for this fusion hero to live up to his potential.
The debate on who is Sentry is a reality-shaper with the power to subconsciously manipulate molecules. When these two powerhouses combined, during the Annihilation: Scourge, event, the result should have been pure, condensed awesomeness.
In Annihilation: Scourge, the Sentry's evil alter ego, the Void, becomes the herald of the forces of the Cancerverse and invades the Negative Zone, to use it as a foothold to invade Marvel's main Earth-616 universe. This happens because Robert Reynolds actually separates himself from the Void in an attempt to get rid of his dark side. Reynolds is then left powerless and stranded in the Negative Zone, where he is found by Silver Surfer in Annihilation: Scourge - Silver Surfer, by Dan Abnett, Paul Davidson, Matt Milla, and Clayton Cowles. At the time, Norrin Radd had also experienced some dramatic changes: after a momentous trip to the past, Surfer was left with an intangible body, which limited his abilities. To stop the forces of the Cancerverse and the Void, Surfer asks Reynolds to merge with him. They are two incomplete beings who were destined to meet: "two half lives into one whole". The resulting fusion is a combination of Silver Surfer and the Sentry, theoretically powerful enough to stop the Void and the Cancerverse.
However, what actually happens in the finale of the saga is that the forces of the Cancerverse are stopped by Richard Rider, aka Nova, who (momentarily) sacrifices his life to stop the threat. The only thing that Surfer-Sentry does is arrive on the battlefield and make Reynolds merge with Void again, bringing the dangerous creature under control. It's an incredibly underwhelming result after so much build-up. There is a huge, cosmic battle going on between an army of galactic heroes and the Cancerverse, and Sentry-Surfer just stands there. If Silver Surfer's only purpose was to deliver Reynolds to the battlefield so that he could merge with the Void again, then there was absolutely no reason for their fusion.
However, Marvel has brought similar characters back after decades, so the Sentry/Silver Surfer fusion could still potentially make a return. It would only take the explanation of their bond creating a cosmic connection that is reactivated later for fans to actually see what the Sentry-Surfer can do, or else for some time or reality-warping event to reach back in time and involve them in another conflict between Annihilation: Scourge's story beats (again, something that has plenty of precedent in the Marvel Universe.) The hint that Silver Surfer and Robert Reynolds were destined to meet even lays the groundwork for their shared form to return, as conveying Bob to the Void is nowhere near enough to justify this sense of epic destiny.
Silver Surfer and the Sentry are two of the most powerful heroes in the history of the Marvel Universe, and while their combined form was a missed opportunity, future stories may turn their merging into only the prologue to a later adventure that finally does the idea justice.
