Robert obtained a device called a Confluctor that gives him the ability to access Doctor Strange's pocket universe that he created inside his mind. In that universe he could act as Sentry without running the risk of unleashing Void in the real world. During the day he is just Robert Reynolds with a mundane life while at night he was Sentry along with his family (Scout, Sentress and Watchdog) fighting crime, every 24 hours he must return to keep Void at bay in this mindscape.
After returning from work, Bob discovered that the confluxer had been stolen and killed Scout and Watchdog. The real culprit was his former real-life partner Scout who had allied with an old enemy, Cranio, to supplant him. After transforming into Sentry in real life, Bob was teleported to Sentry World where he met Cranio who told Billy about it and injected himself with a recreated version of the Sentry Serum to fight the real-life Sentry.
This broke Bob and he decided to accept the dark side that was tormenting him, he was compelled to go directly to his enemy, Void, who needed his help. Bob merged with The Void who became a dark version of Sentry, killed Billy who was already suffering from the effects of the Serum and stopped Cranio. He went into space after a short encounter with the Avengers who wondered if he was on their side.