Based off of this thread by Tyzuris:
Only in this case, he doesn't get a fleet or anything. He's just reborn so to speak on some random overlooked frontier world of Humanity and when he's of age (which should be a few months) he gets full knowledge and memories of his past life and misdeeds (which he will atone for) and the idea of redemption to save these Humans from 'the Trials to come' without any actual foreknowledge of the Covenant (or things like the Flood or Forerunners etc etc etc).
So pre-Chaos corruption Horus gets resurrected with his memories up until his death by Emperor. He gets resurrected in Halo-verse a few years before the start of human-covenant war. ROB says that in order to atone for his misdeeds, he has to save these humans from the upcoming Covenant threat and lead them to victory over the Covenant forces that shall arrive in a few years. Horus is also given his legion of space marines. Horus realizing his big fuck up and feeling his guilt and regrets over betraying the Emperor accepts the task.
Can the Warmaster lead UNSC to a glorious victory over the xenofilth Covenant?
Only in this case, he doesn't get a fleet or anything. He's just reborn so to speak on some random overlooked frontier world of Humanity and when he's of age (which should be a few months) he gets full knowledge and memories of his past life and misdeeds (which he will atone for) and the idea of redemption to save these Humans from 'the Trials to come' without any actual foreknowledge of the Covenant (or things like the Flood or Forerunners etc etc etc).