Priority: Thessia

Acquisition
After the Priority: Rannoch was complete, the Asari Councilor will contact you, meet her at the Earth Alliance's office in Citadel Embassies sector.

Walkthrough
Liara is a compulsory squad member in this mission. As soon as you leave the shuttle move down the pile of rubble towards a bridge and talk to Lieutenant Kurin. Just as she begins to speak there is an explosion on the bridge and most of their barrier is destroyed. She tells you to take control of a gun on the right side of the bridge. A lot of Brutes appear but are easily killed with the gun, what you need to watch out for are all the husks that will climb over the side of the bridge, since everyone else is apparently useless they can easily chip away at the barrier if you don't pay attention. It's also advisable to take out the barrier machine that's just where the brutes begin to appear on the bridge. After you get rid of everything on the bridge a cutscene starts and Lieutenant Kurin tells you she's been ordered to hold the area but she's going to get her people out of there instead but Shepard and Liara manage to convince her to hold the area. Now 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, finish off the Cannibals and continue to the other side. Just through the entrance there is a path to the right, continue and you'll see a harvester attacking and killing some Asari. You'll be attacked by Marauders and Cannibals again. Just make your way across the opening slowly taking out all the enemies as you go and watch out for the few spores there are scattered around. Liara will tell you there's a door just ahead and a Banshee will appear accompanied by more Marauders. Stay behind cover and try to get the Banshee to concentrate on your teammates and take it out first. Get rid of the rest of the enemies and continue through the door. Move along the bath and bypass the door you come too.

As you come through the door the camera centers on a Reaper in the (not too far) distance. Start shooting at the cannibals before they notice you and make your way over to the Asari snipers on the balcony across from you by walking across the opening and up the rubble on the left side. (There is a M-98 Widow on the left of the balcony) Now take out as many enemies as you possibly can, note that there is a barrier machine on the wall opposite you. Once the coast is clear, proceed down the ramp to your left, there should be plenty of ammo scattered around should you need it. Go through the hole in the wall and to your left. After the cutscene you'll have quite a few enemies to deal with, 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. There is a missile launcher at your feet. You're probably going to need it. I advise moving back down off the platform (the way you came) and taking it out from there, moving further back 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. Husks will attack you here, not many but don't let your guard down as in a moment you're going to be attacked by two Harvesters accompanied by other enemies. Just stick to cover and take shots at them whenever you can and watch out for any Swarmers. Thankfully you get a little air support here but you're probably going to have to move back a bit again, this makes it easy to take out the Harvesters first, then get rid of the Maraurders since they'll keep throwing up barriers and then get rid of the remaining enemies.

Continue forward and round to the left, there's a barrier control Liara can bypass to the right of the entrance. Make your way into the Temple of Athame, the scientists bodies are lying on the ground at the front of the middle statue, examine the statue and their cutscene will begin. It turns out that the Reapers are not responsible for the dead scientists. 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. As usual, he's right. You will have to examine other artifacts again for connections. Once you're done the statue will crumble revealing the Beacon. Activate it. An AI appears, ask it whichever questions you wish. It will detect someone who is indoctrinated, Kai Leng appears to deliver a hologram of the Illusive Man. He wants to control the Reapers instead of destroying them. After a short conversation you'll have to fight Kai 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 since he's very fast. Once you take out about half of his shields a gunship will cover him. He won't recharge fully but this process with continue. 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 start and the gunship targets the supports of the building. Shepard will fall below along with most of the floor with Kai Leng concentrates on the Beacon. The camera pans back to reveal Shepard holding onto the side and climbing his way back up. Kai Leng manages to get away with the information he came for.