No quest.

The Legion starting quest in Dalaran should become available after you have finished the entire pre-patch quest line. You only need to have finished that quest chain on one character to proceed.

On alts, you either need to finish the stage you're currently on and Khadgar's servant should then spawn and offer you the Legion starting quest in Dalaran.

If you have yet to finish the pre-patch series, you will need to finish that questline on at least one character first.