Once you have talked to Arutha after Immam was kidnapped he gives you coordinates to go help save Immam and says Adnan is waiting for you their.
Fly to the coordinates and find Adnan in Fionas Arc. Once you find him his second quest will activate. Once you complete the second quest the main part of this quest activates.
Adnan gives you a submarine to use as he sends you to go find his brother somewhere in the deeper levels of Fionas Arc.
Instead of finding his brother you find Mad Fiona in the deep parts of Fionas Arc. Dialogue plays and you discover she has killed Immam.
Kill the Mad Fiona. Once the Mad Fiona has been killed she will turn into her second form which is Golem Fiona.