What quest in wizard101 gets you into nightside?

You'll need to complete the three areas in Olde Town (Firecat Alley, Trition Avenue, Cyclopes Lane). After that, the Headmaster will summon you to battle Foulgaze. After defeating him, a key will be dropped and this will gain you access to the secret door leading to the ruins of the Death School under the waterfall over the Unicorn Way Bridge. When you finish that quest, the Headmaster will send you to fight Nightshade. If you defeat him and possess a membership, the Headmaster will then give you a spiral key where you can travel to Krokatopia and continue your adventure.