Siren quick guide (sorry for spelling errors)
-Talk to Levil to start quest
-Go to Eastern Adoulin (Castle adoulin gates) to speak to Arciela for next Custscene
-Go to Escha Ru-Aun, make a left and click on the glowing spot (same custscene trigger from Rhapsodies of Vana'diel)
-Report back to Arcielia
-Speak with white Dragon in Kamihr Drifts
-Go to the telepoint related to your home nation (Windurst - Mea ) and clock on the Ergon Locus under the stairs for a Key item
-Go to leaffalia and click on Drifting Feather glowing spot for custscene
-Click on Drifting Feather agian for a battlefield with Siren
Defeating siren gives usual gear, gil and ability to summon rewards so I assume BC is repeatable.