This is a Mission Expansion suggestion that is "Free 2 Play" friendly. This mission expansion can be bought with Crystal Energy.

A soggy looking Stranger has appeared near the fountain in Haven, telling tales of a part of the clockworks long forgotten by all. All except King Tinkinzar. The Stranger tells the Knights of a city, a lost city said to be home to many wonders. The city was home to a lost race, said to have sealed away a powerful secret in the heart of the city, causing the entire city to sink into the ocean it floated over. Now a mere fragment in cradle, the city rests within it's very depths constantly switching positions. However, the city appears to have shifted underneath Haven and an opportunity has awakened. The Stranger taps a few stones on the fountains, and to many of the Knights amazement a secret cavern opens beneath the fountain, revealing a hidden elevator that will take Knights below to the very fragment of a once vast ocean.

However, it would not be as easy as they thought. The Gremlins got to the elevator first, rerouting it to a secret aquatic laboratory that they use to send daily missions into the deep blue ocean. Due to the high Gremlin activity, and general lack of care for nature, the ocean has been severely polluted. Mutating much of the undersea life. The Knights of Haven must now embark on an adventure not only to save the ocean, but to prevent the Gremlins from obtaining whatever is sealed within the great sunken city.

This mission works exactly like the Crimson Hammer mission line, players can run through the mission in T1, T2 or T3 form. This expansion has 5 levels total and a few new mechanics and enemies.

Floor one:

Gremlin Labs

Knights must battle there way through a Gremlin themed area, consisting of thwackers, menders, demos and knockers. This level layout is nothing really special, just your basic Gremlin area designed to look like a lab. Players will notice scuba gear scattered throughout the area, and strange tanks filled with aquatic lifeforms that have never been seen before. The elevator going down will also be submerged in a small amount of water.

Floor two:

Dangerous Depths

This is where the real challenge starts!

While the Knights were charging through the research lab, they noticed Gremlin scrawlings on the various chalkboards that indicated a lack of oxygen down below. The knights picked up strange devices that hooked up to their headgear, allowing them to breath underwater without worry.
However, physics under the ocean don't work like physics on land. Knights will now notice that their attacking speed and movement speed has now been lowered! Knights in underwater levels are under a constant debuff that lowers attacking speed and movement speed by a "low" amount. Players will notice "Jelly" fish that can deal a "shocking" amount of pain. These water based Jellies will do a spinning attack that not only inflicts pierce damage, but can also shock. A new type of Gun puppy and Gremlin are also introduced in this level! Gremlin Divers will appear throughout underwater segments, jetting around underwater with some sort of air jetpack. These Gremlins will stop, face their target, then fire a medium sized harpoon from a harpoon gun that travels at a decent speed. The Gremlin will quickly pull out a harpoon from a holster and reload it's gun before moving or firing again. The next new enemy is the Bullshark puppy. These gun puppies spawn underwater, with their shark-like heads they fire out a pulse of water that travels at a slow speed, but becomes faster the longer it travels. These projectiles can inflict poison at a low rate.
Knights who travel through this area may notice strange plant-like growths on the sea floor that randomly pop out of cracks. You should take care not to step on these, as these growths will give you a nasty jolt of electricity!

Floor Three:

Graveyard of Galleons

This next section introduces another new enemy, Cuttle-cutthroats. These small squids lack a mouth on the bottom of their bodies, instead they have a rubbery tube like mouth underneath their eyes. These squids also dual-wield sabres, and posses a nasty spinning attack! If that wasn't bad enough, they can spit toxic ink blobs through the water like grenades! Ink blobs will fly a set distance, then cause an extremely small explosion when they either collide with something or reach their max flight distance. These ink blobs can inflict poison, while the spinning attack will simply knock away enemies. Both attacks deal pure normal damage.

The brave knights of Haven will now be treading through a ship graveyard, members of the undead waking at their very presence and attacking. Spookats, zombies and Cuttle-cutthroats will spawn here, along with Bullshark puppies. As knights fight through this stage, they will enter a large galleon that was sunk long ago from a fierce pirate attack. This half of the stage will bring Knights out of the water, restoring their speed temporarily.
After fighting through the galleon, knights will come to one last party switch in the captains quarters. Knights must now fight a combination of Zombies, Cuttle-cutthroats, Bullshark puppies and one Phantom! After achieving victory, knights will proceed out of the ship and onto an elevator. Taking them to the very city they have been searching for!

Floor Four:

Sunken City plaza

The knights arrived at the sunken city, only to discover that some portions of the city are covered in large bubbles. This level is half aquatic, adding one final enemy to the fray. This enemy is the City guardian, a large Trojan wielding a trident! These Trojans are 1.5x bigger then regular trojans, and slightly slower in attack. However, they deal split shadow/pierce damage! This level contains all of the previous enemies, containing sections of under water warfare and above water carnage! Knights will battle to the city heart, and board the elevator to their final target! However, much to the Knights horror, the Gremlins got to the city heart first. And have broken the seal and unleashed an unspeakable horror!

Floor Five:

Briney Brawl

Upon reaching the city heart, the city will be having violent shaking spasms. Knights will notice a lost spirit floating gloomily near the heart stations. Talking to him, reveals more of the city lore. The ancient citizens used to live on a peaceful island, however a band of aquatic pirates constantly attacked their island, led by a powerful sea overlord known as Captain Davy Krakun. The civilization sacrificed itself to give the sea everlasting peace, sealing the pirate overlord within the city heart and sinking the city to the depths of the ocean. The squid monsters that have been roaming the ocean floor have been trying to breach the city for years, but never had enough strength. The Gremlins foolishly opened the seal and unleashed the Pirate Overlord himself and now our brave Knights must stop him before he attempts to take over cradle!

Once players are ready, they will entered a square arena surrounded by water. Once the party switch is flipped, a large sword will burst from the bridge behind them, completely destroying it and cutting off all hope of escape. A large squid will arise from the water, wearing a large pirate hat. Captain Davy Krakun will reveal himself, four giant tentacles emerging from each side of the arena. Two tentacles will wield sabres, while one has a large pirate hook. The final tentacle will have a very large and rusty cannon chained to a large piece of wood, in an attempt to make a giant gun. Davy Krakun will then submerge his face in water, bringing his hat to the players level. The only way to harm Davy is to attack his face, however you must defeat all four tentacles before you can expose his face! Each sword will target certain players, and attempt to slam down onto them. However, these tentacles have an extremely short range but make up for it by being quite fast and unpredictable. The hook however, can hit anywhere on the arena. However it is extremely slow and is more predictable then the sabres are. Finally, the cannon will be fired upwards and will rain down cannonballs that will explode when they collide with the ground!

After players defeat all four tentacles, Davy Krakun will raise his head out of the water and will be vulnerable to attacks! However, he is not completely defenseless! He will shoot out a large spreadshot of ink torpedos that will poison anyone they hit! This attack has a cooldown however, so players need not worry about him spamming this attack. Davy Krakun will eventually spit out a poisonous mist near his face, poisoning all players in the mist. Then he will submerge his face again and bring all four tentacles back into the fight. Players must then repeat defeating the tentacles to expose Davy's face as many times as needed.

Once Davy Krakun is defeated, he will sink into the water and the arena will magically repair itself! Sealing up the water with rock, allowing the Knights to pass. Another lost soul awaits near the elevator to congratulate you, and gives you a strange dubloon as thanks for defeating Captain Davy Krakun. Players may now leave and claim their rewards!

Upon completeing the mission for the first time, players are awarded three items!

Mystic Trident: A trident that works like a pierce sword. Deals split pierce/elemental damage. Final thrust fires out a small ball of water that disappears when it hits something, dealing pure elemental damage.

Harpoon gun: This gun was swiped from the gremlin labs, it is a prototype handgun that fires small harpoons! This gun fires three shots before needing to be reloaded and deals pure pierce damage. The charge attack fires a single piercing harpoon that flies through the air with a shark head silhouette.

Briney Bomb: An elemental blast bomb that explodes with a dark blue explosion with a jolly roger shown in the explosion. This blast bomb has low knockback.

The stranger in Haven will also make you two new shields in exchange for dubloons and Energy! (Along with a few extra materials of course!)

Shell Shock: A shell of a forgotten sea creature. An explosive rage builds within knights when they wield this shield. This shield will provide Bomb charge time reduction: Medium and Bomb damage: Low.

Crest of Neeru: The crest of a forgotten city. It swirls with blue energy. This shield is a blue version of swifstrike, however the bonuses it gives are different! The 5* version of this shield gives Gun damage: Medium and Gun Attack Speed Increase: Low

Each shield provides Elemental and shadow defence, along with resistance to shock.

And that's my idea. I figured a "F2P" expansion mission would be nice.