Step 1. Ensure that you’ve built your city on an island they once ruled. 
Step 2. Use the Crystal that bound them to the Underworld. (It’s behind the Nine Thrones. You can’t miss it.)
Step 3. Have a traitorous second-in-command who has summoned one before.
Follow the above steps to have an 18 foot, rancid-breathed demon at your command.
One thing I should mention, before you go to draw out the summoning pattern…
DO NOT let the demon take the Crystal from you. Really. I mean it. That’ll get you into a whole world of trouble. Even more than screwing around with your former second-in-command. Which you are not going to do… Are you?
Right, as long as that’s understood… and oh…
Happy Summoning!
KIM KNOX 
heheh
love it!