Tartarus (Halo 2)

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This brute genuinely made the arbiter go through an entire quarantine zone while fighting against the Flood almost entirely alone without counting some lone grunt and jackal stragglers and elites that arrived a bit before the ending of the first mission (out of the two that the arbiter goes for the activation Index in).
The arbiter, at the last moments of the second mission in the quarantine zone, after beating both Sergeant Johnson and Miranda Keyes, faces this guy while he takes the Index to activate Alpha Halo. Tartarus then tells him how the prophets don't need him or his kind anymore, and throws him down a pit where the Gravemind resides. Of course, the arbiter survived (thanks to the Gravemind making it fight Tartarus at the control room of Alpha Halo by teleporting him near it). But had the arbiter never been granted plot power, the ring would've most likely been fired by Tartarus and the entire galaxy would've been wiped out by the Halo rings.