From Halopedia, the Halo wiki
Staff Sergeant Mobuto participated in the Fall of Reach, and was on the UNSC Pillar of Autumn when it made a blind Slipspace jump to escape from Reach. Since the Pillar of Autumn traveled directly from Reach to Halo, it can be assumed Mobuto was involved in the battle itself. However, his involvement during the conflict is unclear and unaccounted for. He participated in the defense of the Autumn and was evacuated before the craft crash landed onto the Halo.
While on the ring, Mobuto was somehow detached from his squad and was contacted by 343 Guilty Spark, who used Halo's teleportation grid to transport him to the Library, much like he did with John-117. Instructed to retrieve the Index, Mobuto fought valiantly through hordes of Flood, Combat Forms, Carrier Forms, and Infection Forms. He surprisingly managed to survive until he was in sight of the Index. However, with little experience against the enemy and standard, UNSC-issued ballistic body armor as protection (which 343 Guilty Spark referred to as "insufficient armor"), Mobuto eventually succumbed to the Flood's might. His body was completely thrashed and mangled to the point where even the Flood Infection Forms could not make use of it.
John-117 found Mobuto's body in the Library during his mission to collect the Index. John discovered Mobuto's identity through his dog tag and paid his respects to the Marine. He was surprised at how far Mobuto had managed to get, as he himself had been pushed to the limit to get as far as he had even with his augmentations and MJOLNIR body armor. Mobuto's determination boosted the Spartan's morale to continue through the Flood-infested halls and complete his mission.
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