XCOM 2

Story-related and cannot be missed.

This is one of several story-based missions. To access this mission, you must first have Dr. Tygan research the Codex brain (see A Grim Key). As with the other story-based missions, this one has a lot of enemies and is guaranteed to have at least one Gatekeeper (one of the most powerful enemies in the game). These look like gigantic metal orbs with fleshy tentacles inside. Gatekeepers have a lot of health so it is unlikely you will kill them in a single turn. The best strategy is simply to Dominate them with a Psi Op unit, which you should have at this point in the game. Otherwise, use a Grenadier to shred its armor and make use of Flashbang/Gas/EMP Grenades. Don't cluster your units too close together, as it favors using its Gateway attack. Not only does this attack do a lot of damage to a large area but it turns any humanoid enemies (dead or alive) into psionic zombies.