Hello again, young wizard. So, you cut your teeth in Unicorn Way. You braved the flames of Firecat Alley. You brokered peace with the Gobbler King. Well, Savior of Wizard City, what's next?
After defeating Lord Nightshade, you will discover a place called Nightside. The door to Nightside is hidden behind the waterfall near the Rainbow Bridge in the Wizard City Commons. Apparently, the Death School fell into a ravine! Here, you can learn the remaining Death School spells and some taunt/de-taunt spells as well. Further, the entrance to the Sunken City is in Nightside. Marla Stinger needs to prove that Grubb exists, and you are tasked to help her.
Sunken City is the first "dungeon" in Wizard 101. What's a dungeon? Dungeons are instanced/private zones in Wizard 101 which are more difficult than the other areas in the same world. When you are about to enter a dungeon, you are prompted with a warning which tells you that the area will reset if you are gone for 30+ minutes and approximately how long it takes to complete. Sounds like fun!
Always have your health, mana, and potion(s) filled before entering a dungeon. Always make sure you are "friends" with everyone joining you in the dungeon - this makes life much easier as you can warp to them if you fall behind or are defeated. Setup your deck to fight groups of monsters (AoE spells), and leave the minion spells out if you have a full group of four. With Sunken City, you really want to finish-up your Wizard City quests and travel on to Krokotopia first. Why? The quest that leads you into Krok gives you another potion. This will help more than you think in Sunken City.
Not in Kansas Anymore...
Sunken City doesn't mess around. I thought Wizard 101 was too easy, until my wife and I duo'd Sunken City at level 14. The sidewalks are no longer safe - the monsters will pull you from quite a distance sometimes. For the street fights, expect 2 enemies + 1 enemy per extra player character. Further, the typical "street mob" has ~500 health (compared to ~175 health in Wizard City). Needless to say, Sunken City is not for the feint of heart.
(Note: my experience was strictly solo or duo - the number of enemies may change if you have a full group.)
Marla Stinger promises to meet you inside the Sunken City to begin your search for Grubb. However, she's not right inside the entrance. A batch of street minions will jump you before you have a chance to say "is your homework assignment *this* important"? After you dispose of the minions (or learn the hard way that you're just not that tough), Marla Stinger sends you on your first quest within the dungeon. Simple enough - figure out how to open a gate. This will bring you to a nearby tower where you must face-off against 3 elite wailing wraiths (death). Be sure to read the book (Tome of Decay) within the tower before you exit.
Next on the quest list is Paulson. Enter the next tower to defeat Paulson (ghost) and his 2 minions. Paulson is a Death School boss, so his resistance to all death spells is jacked far higher than death minions/elites. Don't even attempt to reduce his 800 health to 0 with death magic. You could use life magic, but Paulson and his minions pack life shields. You're better off using any other kind of magic. Defeating Paulson will give you the "Knock" scroll, which opens the second gate.
This brings us to the most grueling part of Sunken City - Norton's tower. Welcome to 5 floors of Heck. This marathon doesn't play nice like later end-stage towers (Big Ben, for example). Norton's tower has no health or mana globes scattered about to help you on your way. Nope - you are tasked with making it all the way through the tower without reprieve. Completing the Golem Tower in Wizard City at a relatively low level will prepare you well for this marathon. Try to end each fight with a fair amount of health and save one potion for the last floor. The first floor contains 3 Wandering Specters (500 health, rank 3 elite myth). After that is an easier fight against 1 Wailing Wraith (death) and 1 Wandering Specter. Up next are 2 Wailing Wraiths. On the 4th floor, you will encounter 1 Wailing Wraith and 1 Living Scarecrow (675 health, fire). The final battle against Norton (Balance Boss) and his 2 Wandering Specters would be a tough one normally - but, you are asked to do it after 4 floors of dueling! Balance has no opposite school, so it becomes a straight-up slug-fest. Hopefully, you have enough ammunition! Defeating Norton will give you the Skeleton Key, which is needed to gain entrance to Grubb's castle.
After Norton's tower, you make your way to Grubb's castle. Grubb is a rank 3 death boss with 1,000 health. As with Paulson, don't even bother with death spells against Grubb - his resistance is just too high. Grubb is accompanied by 2 Living Scarecrows, so this fight isn't exactly a walk in the park. Try to kill the scarecrows first and then take down Grubb - less total enemies is always easier to deal with. Plus, Death bosses can life drain, so you may as well save Grubb for last.
Congratulations! You found proof of Grubb's existence and validated Marla's homework assignment. More importantly, you can wear the title of "Sunken City Survivor" - you earned it!
Tips & Tricks
After you defeat an enemy on the street, that enemy will remain defeated - there is no respawn!
I've read numerous posts about Sunken City around the internet, and some people believe you cannot warp to/from Wizard City while you and your friends are in Sunken City. This is wrong! As long as one of your friends remains in the dungeon to "hold" it, you can freely warp back and forth to Wizard City to refill potions and gain health/mana. Use this to your advantage!
If you are defeated in a close fight and your friends don't know if they'll pull it off, FLEE! Drink a potion, and warp right back to the fight. You might just save the day.
If you are a seasoned MMO veteran, Sunken City will probably be your first major challenge in Wizard 101. Finding a reliable group could be troublesome, but the badge is worth the effort. Don't expect to be challenged at this level again until Marleybone, Kensington Park.... but, that is another story.