Priority: Thessia

Acquisition
This mission is acquired automatically after completing Priority: The Citadel III by speaking to the asari Councilor in the late Donnel Udina's office.

Preparation
Liara is a compulsory squad member in this mission. As such, one of the best things you can bring is Overload, Energy Drain and other anti-shield abilities which render Marauders vulnerable to Singularity and/or Warp. Another great priority is raw damage, which Liara simply doesn't provide a whole lot of. But if you're interested in some background explanation on asari history, consider bringing Javik.

Walkthrough
In the first area, there's an Umbra Visor and supplies worth 4,500 credits towards the right, away from Lieutenant Kurin. Grab it before you do anything else, because speaking to Kurin will cut off your access to the starting area. When the fighting begins, she'll direct you to grab a turret gun. A lot of Brutes appear, but the real threat are a Barrier Engine near where the Brutes are spawning which essentially doubles their HP, and the husks that sneak towards the biotic bubble on the far left.

After a cutscene, it's time to cross the bridge. A bunch of Marauders and Cannibals flood towards you from the other side of the bridge. As always take out the Marauders first. The other side looks like a dead end, because the path is to the right, blocked off visually by the wall. Continue for more Marauders and Cannibals, as well as Spore Pods. After clearing to the Marauders, go to the far left to pick up a M-560 Hydra missile launcher. As you move a little further up, Liara tells you of a door just ahead, but you need to deal with the Marauders and a Banshee first. The Hydra will be very helpful against them. Fortunately the Banshee appears at a distance, giving you more time to damage her. Before you leave the area, you can pick a med-kit on the far right and left of the area.

Walk along the path while salvaging the Terminal worth 7,500 credits and a bypassing a door. You'll get a camera prompt for a Reaper in the not-too-far distance. You'll also see three Cannibals crouching in the courtyard below; with the right abilities, you can kill them without firing a shot. Just to the right of the cannibals is a broken shotgun that can be salvaged for 3,000 credits. Directly above the broken shotgun on a small ledge is a Med-kit and Shotgun High Caliber Barrel. Make your way over to the asari sniper and snag the med-kit, Sniper Rifle Concentration Module and M-98 Widow before talking to her. Now take out as many enemies as you possibly can, including the Barrier Engine on the wall opposite you. The asari sniper will provide a constant stream of moral support, though her bullets may not be as effective.

Once the coast is clear, proceed down the ramp to your left and collect ammo if necessary; there should be plenty. Go through the hole in the wall and to your left. After the cutscene, be careful of the Banshee as it can get ridiculously close within just a few seconds. Also take note of a Ravager just up the stairs, and a Barrier Engine on the far left, hidden by a partition. There is a M-560 Hydra missile launcher at your feet; for optimum use against the Banshee, retreat back to the way you came from and even further if necessary. Thankfully you'll have a little help from a gunship.

After you get rid of all the enemies continue straight forward up the rubble and then to the right. Near the Spore Pods is an Assault Rifle Extended Barrel pickup. Past them, Husks will attack you; past them, you'll tangle with two Harvesters accompanied by Marauders, Ravagers, and--way in the distance--a Barrier Engine, just to make your life that much more difficult. Stick to cover, take shots at them whenever you can and watch out for any Swarmers. Consider falling back, which makes it easy to take out the Harvesters first, then get rid of the Marauders since they'll keep throwing up barriers and then get rid of the remaining enemies. Also be aware that you don't actually have to kill the Harvesters: once Talon Five (may the Goddess bless her soul) goes down, they'll bug out.

Continue forward and round to the left to find a SMG Heat Sink and a barrier for Liara to bypass, on the right of the entrance. Make your way into the Temple of Athame, picking up a med-kit close to the entrance. The scientists' bodies are lying on the ground near the center statue, which (coincidentally) is the one you're looking for. Look around at all the artifacts inside the museum, if the prothean is with you at the moment, he will give further explanation on the artifact source, better then Liara. As it turns out, Shepard thinks there is a Prothean Beacon in the statue, it would explain why the asari are so advanced and why scientist had been studying the temple for thousands of years. You will have to examine other artifacts to expose the beacon. It produces a VI, Vendetta; when you're done conversing with it, it will detect someone who is indoctrinated. Kai Leng appears to deliver a hologram of the Illusive Man, who (as usual) wants to control the Reapers instead of destroying them. Regardless of what you say to him, you'll have to fight Leng.

Stay behind cover as he sends shockwaves that disable your shields. You'll need to move around a lot and use your squad to distract him as best you can. Once you take out about half of his shields a gunship will cover him while he recharges them. This will repeat several times. Just keep as much distance as possible and make sure to stay behind cover when the gunship starts shooting. When you get his barriers down, a cutscene will resolve the rest of the battle.

By the end of this mission, you will gain 15 Reputation, 15,000 credits and morality points depending on your conversation options.

Enemies

 * Banshee
 * Brute
 * Cannibal
 * Harvester
 * Husk
 * Kai Leng
 * Marauder
 * Ravager
 * Swarmer

Aftermath
Specialist Samantha Traynor tells Shepard that she tracked Kai Leng to the planet Horizon in the Iera system, making the mission Priority: Horizon available. The side mission N7: Communication Hub will also become available.

If Javik has been recruited you can witness a confrontation between him and Liara in the Port Cargo Bay on Deck 4 and have a conversation with Javik afterwards.

If Tali and Garrus are alive then Tali will be standing in front of the memorial wall on Deck 3 talking to Garrus over the radio and can be interacted with for a reputation bonus (and again after talking to Liara for more Reputation).

Liara will be in her cabin and can be engaged in conversation.