Phlogiston is a location in Tyranny.
General Walkthrough and Tips
First, you need to let the scarlet chorus in this area allow you to speak with the captured sages. Then interrogate Phoibe, see what is she really after here. (Eb can help if she's in your team)
You can then decide to kill the sage or let them leave, if you decide to spare their lives. You need to kill the scarlet chorus or trick them to let sages leave.
FInd the Tome in a wagon wreck.