As players soon experience in Divinity: Original Sin 2, Larian Studios fleshes out Original Sin's world of Rivellon with creative use of self-contained sidequests with bursts of the grandiose scale of the main plot. For instance, in Act 2, sidequests eventually point players towards the Blackpits mines in the Reaper's Coast.
What first seemed like a Magister mining operation blew up to be the excavation of an Ancient Temple. Players who get to the center of the underground alien complex will encounter the Eternal Aetera, a being that seems to be just as old as the gods. Unfortunately, fighting the Aetera head-on seems like an impossible task in the Temple's tight space. However, it's perfectly possible to dispatch the Aetera with the right approach.
10 Study Your Enemy
After the first few encounters, the Aetera seems a bit too OP given her repertoire of skills. However, players might be surprised to learn that the Aetera seems similar to most Undead when examined. In her section, she has a permanent Floating status as well as Immunity to Poisoned, Bleeding, Drunk, and Suffocating. Meanwhile, she has complete Resistance to Poison (100%), major Resistance to Air (80%), and some Resistance to Water (40%).
Players could safely use Geomancer Skills that don't deal Poison, and Pyrokinetic Skills. Likewise, it seems Warfare, Huntsman, and Scoundrel Skills can work provided they're not geared towards Bleeding and Suffocating. On the other hand, her Eternal Stalkers are meatier in comparison but interestingly have drastically-low Magical Armor.
9 Assess Starting Positions
Despite the sheer irritation of this boss fight, the underground Eternal Temple seems like a battleground that players can easily use to their advantage. As with most battles, strategies might change depending on starting positions. Thankfully, the dialogue between the Sarcophagus' opener and the Aetera gives the party time to prepare.
The Aetera spawns on the platform on top of the Sarcophagus where the Temple entrance lies. Beside her are two Eternal Stalkers. Meanwhile, two more Eternal Stalkers flank the character speaking with the Aetera. It's crucial to determine how many characters are left in the room and where they're positioned. Always keep this in mind while reading the other tips.
8 Attacks Hurt Like Hell
The Aetera might have "manageable" base stats (e.g., Health, Physical Armor, Magical Armor) but hurts like crazy with her empowered attacks. The Aetera's abilities include summoning other Eternal Stalkers, casting Rain (Hydrosophist), and using Source-based Skills (e.g. Source Vampirism and Chain Lightning). Her Eternal Stalkers have enough AP to either cast Flight, a crippling Bite, or move around the field.
The Aetera and her minions can overwhelm players as early as the first round. As such, players should plan their approach to end the battle before a second or a third turn.
7 Separate Her From The Stalkers
One of the first things players should think about is separating the Aetera from her Stalkers. It's one thing to tank the Stalkers' many-AP attacks, but it's another thing to deal with the Aetera's abilities entirely. As such, all strategies should involve at least teleporting the Aetera far away from her Stalkers or at least toward the characters who need to attack her.
Stalkers who get too close to those attacking the Aetera need to be taken out of commission. Players can teleport these doggos to farther areas to force them to waste AP just running to their favored position. Characters can also act as bait to keep the Stalkers busy. A well-equipped mage and AOE Skills should tear through the Magical Armor and HP of the Stalkers easily.
6 Bombs Away
Given the Aetera's lack of Resistances or Immunities to Fire, players should also consider just bombing the area prior to attacking the Aetera and her minions. Players can put a Teleporter Pyramid on the ground and have characters stockpile on Oil Barrels to position them in the area. A good ol' Pyrokinetic Skill on that barreled area can deal significant damage to the defenses of the Aetera.
However, despite the added firepower, the Aetera can put out the fire via Rain and will most likely use it during her turn. As such, players should try triggering the bombs with the first character that comes after the Aetera when the battle begins. That way, players have four characters' worth of actions to take her down.
5 The Dogs Act As Resources
Unfortunately, players who dispatch the Aetera's doggos too quickly might notice that she can just produce Eternal Stalkers ad infinitum. Granted, players can use this to their advantage and farm EXP. However, this easily becomes a battle of attrition — the Stalkers do have a ton of AP and powerful attacks.
As such, players should be wary of the strategies the Aetera uses with her Stalkers. For instance, she regularly steals Source from them to empower her attacks.
4 Let The Dogs Do The Work
It helps to mention that the Eternal Stalkers have incredibly low Magical Armor, as there's a way to get them to attack their master instead. Interestingly, players can try gathering the Eternal Stalkers and using a Love Grenade (Empty Perfume Bottle + Pixie Dust or any Honey) to Charm them for two turns. If players eliminate the Stalkers' Magical Resistance with spells from a mage, then the doggos are as good as charmed with the grenade.
However, it's tricky getting the Stalkers in one spot without TPK-ing the party. Players need to do this while tanking the attacks of both the Stalkers and the Aetera. Ideas might include teleporting them to one spot one by one or having one character in the Temple as bait to lure them into one location.
3 Good Old Teleportation
Players might remember that they recently had to dodge some tripwires in the area directly before the Eternal Temple. Interestingly, players can actually use this area to their advantage.
As per usual, one character stays behind to trigger pre-battle dialogue. Prior to the battle, players can position themselves close to the shaft opening on the far left and activate sneak. This allows them to sneak attack the Aetera later on. Afterward, characters with Teleport and a good enough range (e.g., the Far Out Man Talent) can lure her out of the dialogue and into the previous area.
This way, other party members can focus on taking down the Aetera in this location. Meanwhile, one of them can stay behind as bait for the Eternal Stalkers or even just take down the Aetera before the doggos reach them.
2 Some Fatal Beatings
At the end of the day, nothing beats a good beat down. One of the most straightforward strategies to face the Aetera lies in pummeling her to the ground before she can even react. Players should position the tank as close to the Aetera as possible in order to strike her down as soon as their turn arrives. They can do this by positioning the tank(s) beside the Aetera during pre-battle dialogue, or teleporting the Aetera beside them whenever she tries to escape.
To avoid complications, it helps to keep her Knocked Down with Warfare Skills such as Battering Ram, Battle Stomp, and Overpower, as well as the Earthquake (Geomancer) Skill.
1 Attack From Behind
A great alternative to tanking the Aetera would be Backstabbing her through stealth and tanking from there. Essentially, one character stays behind to begin the dialogue and act as bait. Ideally, this character should be able to tank the Aetera and Stalkers' attacks or at least run away from them with Talents such as Duck Duck Goose to avoid Attacks of Opportunity.
Once it's the character's turn, the other party members can sneak up on the Aetera from behind and do sneak attacks. Chances are, the Backstabs should be enough to whittle her Physical Armor. Afterward, the characters should try to keep her Knocked Down to avoid harsh counter-attacks.