XCOM 2

Story-related and cannot be missed.

This is similar to A Grim Key, except now you must use a Skullmine (created from the Proving Ground) on a Codex. Doing so will kill the Codex and summon an Avatar, which is the only Avatar you will encounter in the main game (aside from the final mission). As you might expect, the Avatar is an extremely powerful unit and it is advised to wait on summoning one until you are fully prepared. Do not attempt to summon an Avatar if you are on a timed mission as it may take several turns to kill. Also, do not attempt to summon an Avatar unless the Codex is one of the last enemies on the map. Like the Codex, the Avatar has the ability to teleport and it wreak havoc on your team if you have to deal with other enemies at the same time. The Avatar can mind control your units, has a huge health pool (30 on Commander and 35 on Legend), regenerates health (+5 health every turn) and has several powerful attacks. He will also teleport every time he is attacked.

The best tactic is to spread out your troops so that you can attack the Avatar regardless of where he teleports. Attack with the units that are closest to him first, especially if you have a ranger with a shotgun. Critical hits are great for taking him down quickly. You can potentially kill the Avatar in one turn, so throw everything you have at him. Grenadiers are good for shredding his armor and Psi Op units (especially those with Dominate) are invaluable for this fight. Remember: This is the only time you will have to face an Avatar outside of the final mission so don't be stingy with your consumables.