The quest for hidden messages in Warframe will allow you to obtain Mirage, but you will need to answer a few puzzles first. To access the quest, you will need to complete the Pluto / Sedna junction, then the quest will appear in your Codex.
The quest begins when Ordis discovers a hidden message on an item. Each hidden message is a riddle that tells you where to go. Below you will find the puzzles and nodes that you will need to visit. The reason people struggle so much with puzzles is that they don't have much to do with the game itself, and instead delve into the depths of mythology and legend.
Riddle # 1
Three parts, three acts, three riddles for you -
to revive the one who wields the fantasy
Ancient gods so cruel, once ruled from above
Can you name their summit where the earth meets the sky?
Reply: The Olympus node on Mars. The mission is an Exterminate mission against the Infested.
Riddle # 2
To build the one you think is witty,
look for the nymph who sang oh so pretty.
She caught him with her song and her smile,
his wife must have thought it was a shame.
Reply: The Calypso node on Saturn. The mission is a survival mission against the infested.
Riddle # 3
One more act before your prize is caught.
An ancient sea creature, swirling or not?
Look for the monster that swallowed the sailors of old.
Make a mistake and you're sunk. Nothing more.
Reply: The Charybdis node on Sedna. The mission is a sabotage mission against the infested.
When you get to each location, complete the mission to get one of Mirage's blueprints. None of the missions are particularly difficult, and like the majority of quests to obtain Warframes, they act more like a dumping ground for knowledge than anything else.
You will need to purchase the main model of Mirage from the credit market, as you will not be rewarded during the quest. When you have all the parts, you can build Mirage in your foundry.
