The Silver Surfer and the Sentry once merged together to create one of the most powerful heroes in the Marvel Universe, but the transformation only occurred to stop an even bigger threat in the Void. In Annihilation: Scourge - Silver Surfer #1, the Power Cosmic-wielding hero and Marvel's Superman become one to stop the Sentry's darkest side from destroying the Marvel Universe - resulting in the surprise creation of a silver Sentry.
The Sentry origin is amongst the most tragic in Marvel Comics history. The extraordinarily powered hero made the rest of the universe forget about him to prevent his dark side, the Void, from emerging. With the power of a million exploding suns, the Sentry is one of the strongest, if not the strongest overall, heroes ever. However, his powers come with a high cost, as with them exist the Void, a creature hellbent on destroying everyone and everything. To suppress the Void, the Sentry has erased his memory to stay hidden, but in one of his efforts to split himself from his dark side, he ended up floating across space, where the Silver Surfer encountered him.
In Annihilation: Scourge - Silver Surfer #1 by Dan Abnett, Paul Davidson, Matt Milla, and VC's Clayton Cowles, the Silver Surfer finds Bob Reynolds (in his human form) in the Negative Zone. Reynolds, sent to the Negative Zone by Reed Richards to split from the Void, tells the Silver Surfer he wants to be left alone and die now that his dark side has been unleashed. But, sensing Sentry isn't whole, Silver Surfer reaches into Reynolds, causing them to become one powerful hero.
Together, Silver Surfer and Marvel's Superman become a hero capable of stopping the Void and the Cancerverse's army. The merged heroes wield the Power Cosmic while containing the knowledge and insight to defeat the Sentry's darkest side. In the end, bringing the Sentry back to the battle and merging with him helped the Silver Surfer stop the Void and Cancerverse forces.
Readers didn't get to see the silver Sentry in action a lot, but the concept of the merged heroes was still incredibly cool to see unfold. The combination character's true powers are unknown. However, considering he wielded the Power Cosmic and had the knowledge to defeat the Void, Silver Surfer and Sentry proved to be a formidable, powerful force with an unquantified power limit. It would be a lot of fun to see the Silver Surfer and Sentry become one again in future Marvel Comics storylines - as it could serve as the perfect way to reintroduce the Sentry after his recent death