Nightmare Fuel / Halo 5: Guardians

All Your Base Are Belong to Us.

• The Legendary ending. Rogue Cortana has a Halo.
• It's hard to say which is worse: Cortana is still rampant, or she's not rampant and just mad with power. Either way, when she shows her true nature to Fireteam Osiris, her ranting and sneering is just chilling. Even worse is that she's utterly convinced she's in the right, that the Spartan-IV's and their kind deserve to die and that Chief will agree with her when she wakes him from stasis in 10,000 years.
• The Argent Moon, the derelict ship of the eponymous second mission that Blue Team is sent to recover from Covenant forces. At first, you could be forgiven for thinking that the Covenant found it and killed everyone aboard. But it turns out the Moon was already a Ghost Ship long before Kig-Yar scouts stumbled upon it. ONI scientists had been working on an airborne bioweapon, and it somehow got loose. The result, as hinted by numerous dessicated corpses with an odd residue beneath them, was everyone aboard the entire vessel dying horrifically. According to a record by the shipboard AI, those who were at ground zero for the containment breach (who died instantly as their soft tissues liquefied as a result) were "the lucky ones."
• Hearing the Jackals speak English for the first time in the franchise is a little jarring.
  FAAAALSE ARBITER! KILL! KILL!
• There's an Easter Egg on the multiplayer map "Rig". While the trigger for it is simple ^*  After Ground Pound-ing a platform in the Bottomless Pit in the middle of the map, you shoot two cores coming out of their silos, on the edge of the map., the result isn't. You hear a piercing screech as a rig on the horizon is suddenly lifted from below by a titanic Sand Worm that makes Dune's sandworms look like maggots. One can only hope that no one was on that rig.^{Then again...}  This is just a simulation. Someone in charge of the simulation’s programming was probably messing around. Therefore, it isn’t likely that the sandworm even exists outside of the War Games simulation.
• As Cortana extends her offer to cure the rampancy of all AI who join her, suddenly Fireteam Osiris hears the voices of hundreds of Smart AIs all across the galaxy speaking up to pledge themselves to Cortana. The real scare factor is not only hearing all these voices overlap, but the realisation that a key element of humanity's interstellar infrastructure has turned against them.
  • And the final cutscene makes it clear that, barring Roland who seems to still be on the side of humanity, literally every AI currently active sides with Cortana, dealing a very severe kneecap to humanity on top of having to face off against the Created and the Guardians. And it's an extremely one-sided match, with the Infinity being very lucky to slipspace jump away before the Guardian over Earth could attack it. The crew's forced to keep randomly jumping and stay away from major settlements in order to throw the Created off and avoid drawing attention to other colonies. The deck is extremely stacked against them, with prospects looking very grim.