King Cobra Venom. There are four sections on it, representing the four branches.
The Taj Mahal is a giant tomb built in India by the Janus architect Ustad Ahmad Lahauri for the emperor Shah Jahan
Appearances in the 39 Clues
You have to visit the Taj Mahal in Mission 2. Lahauri hid a poem on a wall saying that he wants and only wants Cahills from the Janus branch to find the clue.