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*Original post. Click at your own discretion.
Move to extract via Pelican as the survivors, complete objectives to progress such as decontaminate an area with flood spawners and restore the power to continue forward. the Flood infect the survivors to win using the various abilities of the Parasite to win before the survivors can be air lifted to safety.
the end scene is one of the Arbiters ships glassing the area the survivors were just in.