Irrelevant Preface: During the events of Halo 3, the Gravemind offers a (very temporary) truce to the Master Chief and Arbiter in an attempt to put an end to the genocidal delusions of the Prophet of Truth. However, the grudging allies fail to prevent the malevolent zealot from activating all of the Halo installations and the Ark. Little do they know, however, that the Forerunners got drunk one night and accidentally rewired the Ark's main computer and instead of unleashing mass murdering waves of doom -- our favorite heroes and villains are transported across time/space to the land known as Nosgoth.
The Gravemind is miraculously deposited in the subterranean lair of the powerful Elder God. The Elder God claims to have forseen this, and both he and the Gravemind recognize each other as planet-consuming parasites with tentacles and countless minions. Time to decide who's the alpha leech!
Combat Scenario: Both of these near-omniscient parasites are gonna slug it out: mentally, physically, and through pawns! The Elder God can summon his supernatural minions to assist him, and the Gravemind can reanimate the rotting corpses in the environment and create all Flood types. However, we'll put a population cap of 20 minions per parasite. The Gravemind also has a cache of assault rifles, plasma pistols, and fuel-rod cannons to arm his people.