To unlock this side mission, you must progress in the main campaign until you return to the Skull Cave in the Hidden City after following the path of the Cross at the Mission of San Juan. Q'orianka will be in the Skull Cave, and you must speak with her. (She gives you "The Age Makers" side mission from The Pillar DLC, but you don't have to perform that mission yet.) You must also complete the "Echoes of the Past" side mission from The Forge DLC. Once you have met these prerequisites, you could
go back to the Mission of San Juan and talk to Abby in the town square to receive the "Eternal Reward" side mission. You won't be able to actually begin
it, however, until you complete the main story campaign, see the credits, and continue your game. So, unless you just enjoy fast traveling from one map to another and back, hold off on going back to the Mission of San Juan until after you've finished the campaign.
The mission does involve a substantial amount of combat with modern Trinity forces. No special preparations are needed, that I can think of.
When you've done the above, speak with Abby. Jonah and Q'orianka will join in and then the mission will begin. SPOILERS FOLLOW. Walk down off the platform and head to your right to find the entrance to the catacombs. Pretty quickly, you'll come to a room where there are 9 soldiers. You can get the first three from the water, but you'll probably lose your concealment and end up shooting your way out before it's over. Continue and shoot your way through a gauntlet of soldiers. Don't try to shoot the guy on the minigun, because you can't - just scramble from cover to cover to advance through the gauntlet. When you hear the guy saying "I lost her! I don't have a shot!" that means he lost you and doesn't have a shot, so boldly proceed forward. You'll see three guys clustered together and be prompted to stealth kill one of them. Surely that won't work, will it? I mean, the other two will be alerted and kill Lara instantly, right? No, actually the cutscene will take over and she will badassfully* handle all three of them.
After a very short platforming section, you will have to solve the Mother Protector challenge tomb. This will unlock:
And that's it. Tell your allies about your success, and you will receive the Silver Strike - Moonbeam
bow and Midnight Sentinel - Farseer
outfit, which you need to unlock:
Please let me know in the comments if I left out something important. That would do more good for the community than an uncommented downvote would.
*Credit to Bubin from "Blasto 6: Partners in Crime" in Mass Effect 3.