Can't Find Keristrasza for Quest Saragosa's End

The quest The Cell awards you with the item Augmented Arcane Prison, which you can use to summon Keristrasza. Summoning Keristrasza will start Saragosa's End, and you can use it again once you've completed the quest to turn it in. 

If you no longer have this item in your bags or bank, speak to Raelorasz at the Transitus Shield. He will offer you a dialogue item to get the item again if it isn't in your inventory.