He was put in charge of the Forerunner Fleet, and used the ships as massive bombs to destroy the opposing fleet. During the fray, he somehow captured Mendicant Bias and took him back to the Ark. He dismantled Mendicant and spread his pieces all over the Galaxy via the reseeding ships, then disappeared, never to be seen again.
Well, apart from 102,000 years later when he popped up at a League meeting. It should be noted that, whereas Mendicant's design was based on a Monitor, Offensive's design was based on a Promethean Watcher, suggesting that the Didact had more to do with Offensive than it initially seems. This would explain Offensive's ridiculous behaviour and bad attitude, then.
His abilities included independent levitation, immunity to Flood corruption, superior combat capabilities compared to a standard Monitor or Construct, and heightened cognitive development allowing him to form his own judgements on the validity of a certain action and reducing his adherence to protocol, an advantage which Guilty Guilty evidently lacked.