The Contaminated Underpath is a solo event instance for players of level 10 and above. It was introduced in the 5.1 update as part of the Luna Shop. This instance is a prime source for Luna crafting materials, but especially experience, granting as much as multiple level-up during lower levels. Other rewards include enchantment stones and consumables among others. As of the 6.0 update it is the first true "afk instance" in the game.
A mad scientist by the name of Dr.Stein has been kidnapping Daevas and other creatures and brought them to the remote Taki's Island, where he subjected them to horrible experiments. Over time, Dr.Stein gradually built up an army of plague-bearing undead which he plans to unleash upon Atreia. Transform into a member of the Luna Detachment and help hold the last line of defense against waves of Dr.Stein's mutilated minions and his ultimate creation: the absolutely hideous .
**As of the 6.0 update this instance will automatically award a 100% S-rank per run. This section can be skipped in its entirety**
Upon entry the player is transformed into a member of the Luna Detachment, and is given 3 minutes to prepare. The instance can be started anytime by opening the gate. Before doing so the floating container should be looted for 3 it contains. Note that these will disappear from the player's inventory if they disconnect or leave the instance.
The goal in this instance is to prevent two barricades from being destroyed. To achieve this the player must use the received skills and construct turrets on the nearby platforms to fend off the incoming monster waves. Points are obtained by defeating monsters, but will deducted each time a barricade is destroyed. Two mini-bosses and a final boss will also appear throughout the run to complicate efforts.
While transformed the player has access to the following skills, which are hotkeyed 1 through 5:
- Soul Skewer - AoE, low damage. Shares cooldown with  Terrifying Rupture.
- Crushing Darkness - AoE, medium damage. Shares cooldown with  Hellish Strike.
- Terrifying Rupture - Single-target, medium damage version of  Soul Skewer. Shares cooldown.
- Hellish Strike - Single-target, high damage version of  Crushing Darkness. Shares cooldown.
- Hellish Wave - Single-target, very high damage. Saved for monsters and .
At the bottom of the stairs the player will see the two barricades that need to be defended. The previously looted are used to construct turrets on the four Defense Turret Platforms located in-between them. Additional can be acquired as the instance progresses. The available turret types are:
- Single Fire Cannon: Single-target damage (low). Costs 1
- Area Antiaircraft Gun: AoE damage (medium). Costs 2
- Single Shot Aethercannon: Single-target damage (high). Costs 2
NPCs/Healing Fountain Edit
Additional defenses include the Luna Guard NPCs stationed behind each barricade, as well as a Healing Fountain which will restores the player's health if they walk through it. The Healing Fountain will disappear once it has been used (or when a barricade is destroyed), and will respawn behind the remaining barricade after some minutes.
Death Zone Edit
The green/slime area ahead of the barricades (indicates by a fire vent) is an instant death zone, and should be avoided. If the player dies inside the instance they may resurrect at the start, and will not be penalized with EXP loss. Soul sickness will still apply.
The frontal barricade will come under attack first. If it is destroyed 5.000 points will be deducted from the player's score. Furthermore, the NPCs stationed behind the barricade will become infected and join the monsters in their assault on the Rear Barricade.
The rear barricade has more HP than the frontal barricade, and stronger guards. If you still allow it to be destroyed another 5.000 points will be deducted from your score, making you eligible for an A-rank only.
Monster Waves (Defense Phase) Edit
Monster waves in the instance are composed of two or three "sets". Each "set" will contain two or three waves, usually subjecting the player to 5-6 waves of enemies total. The monster type each "set" is composed of is randomly selected from the following possibilities, but can be predicted by paying attention to the message that appears on screen:
- Gringol the Devourer or Cursed Brave Warrior (Melee) - "You're hearing heavy breathing/a coarse voice"
- Transfectant Flyer (Ranged) - "You feel a cold sensation"
- Transfectant Crawler (Melee) - "You're hearing lots of footsteps"
- Crazy Priest + Infected Zaif or Experimental Reian (Mixed) - "You're hearing terrible animal screams/a sharp yell"
- [Rare] Infected Saam King - "You're hearing chattering noises"
Gringol the Devourer or Cursed Brave Warrior are fast melee attackers. They come in fewer numbers, but are durable and hit hard. All other possibilities are large groups of weaker monsters, and are best countered by spamming  Soul Skewer and  Crushing Darkness, as well using Area Antiaircraft Guns. Transfectant Crawler provide the highest difficulty due to their vast numbers.
Infected Saam King are a rare possibility that will guarantee a free S-rank.
An added difficulty comes in form of two monsters. These will appear at various points throughout the defense phase: the first will appear throughout the first set, and the second will appear at some point throughout the second set. Players will need to be especially vigilant for the second MAD-74C, as it will charge in while others monsters are already attacking the barricades.
will rapidly charge at the nearest barricade and begin to cast Destruction (MAD-74C). If it is allowed to finish casting this skill it will take 50% off the barricade's health, as well as kill the player if they are standing nearby. The only way to counter it is to defeat MAD-74C by using the  Hellish Wave skill before it can finish casting. A slain MAD-74C can also be looted for additional .
MAAD-S (Final Boss)Edit
After the final monster wave has concluded (or upon reaching the ~50,000 points threshold) a screen caption will appear: "A dark energy is spreading". will appear in the back and slowly draw closer until it reaches the nearest barricade. Turrets (especially Single-Shot Aethercannons) will be a great help in the fight.
MAAD-S will continuously attack a Barricade until it is destroyed. It will use several hard-hitting skills in addition to its auto-attacks: Demolition (single-target damage), Discharge (summons adds), Domination (AoE stun+damage) and Destruction (MAAD-S) (AoE damage).
One strategy to save a barricade is to quickly intercept and spam MAAD-S with skills as soon as it comes within range. It will eventually switch aggro to the player, at which point he/she can kite it around while the turrets and NPCs will continue to damage it. If the player is low on HP they should seek out the Healing Fountain.If MAAD-S is left alone it will just focus on the barricade, although it will still change target to the player after a barricade is destroyed. The amount of time it will attack the player for depends on how much aggro the player has build up (i.e. how many skills they used). Defeating will award the player with 500.000 points and finish the instance.
First-timer quest can be obtained from Luna Agent Eli at the following locations:
will leave behind an Infected Flesh Lump when slain, which when looted will yield a . Seven of these may be brought to the at the start of the instance and exchanged for a , which will unlock the treasure chest found at the bottom of the stairs for an additional reward: . will also leave behind numerous core fragments, which each award large amounts of EXP when busted.
The main reward is one of the following bundles, which is awarded based on the player's total score. These bundles contain Luna crafting materials for use in Karunerk's Workshop as well as a chance at other items.
|Rank||Total Score||Remaining Time||Rewards|
|S||549,000 or above||00:00:00 or above|
|A||100 or above||00:00:00 or above|
|B-||50 or above||00:00:00 or above|
- After the 6.0 update an S-rank will be awarded regardless of score, following the revamp of the Luna Shop. A number of minor adjustments were also made, such as more appearing & at slightly different moments, several new monster death animations, and a decreased duration of the instance overall.
- Throughout the 5.x update completing this instance with an S-rank had a 50% chance of awarding an .