There is alot of stuff. Get ready.
Boss Lore (Doesn't affect the game that much)
“Once the captain of a powerful crew, and motivated by their pride and greed. Venturing far into the Dark Sea in search of riches, they find themselves at each others throats in a attempt to seize the vast riches for themselves.”
"After a long struggle, the captain obtains the treasure, but was abandoned by his crew, leaving him to die. Now being stranded in the sea with no way home, he prays to the gods in a futile attempt to forgive him for his crimes and get him back home. He goes unanswered."
“Days pass as he loses himself slowly in the Dark Sea, making his mental state as weak as glass. Spirits of the dead start to inhabit his body, using him as a puppet to make themselves feel whole again. He remains alive due to them supplying the body with nutrients that they find, but has no control of himself anymore.”
“This is the sorry state you find him in, a husk with this treasure on his face. House to the spirits of the men who died at sea, and to house the cracking soul of the man that perished from there own greed.”
Gameplay
Locating
Finding Him
He would be a rare spawn in the Dark Sea, having the chance to spawn at levels +3 in the sea.
He would have is own unique island that spawns, appearing as a giant cave with markings and pillars on the walls. It would have a pedestal in the middle, with a empty slot for the mask. It would have purple fires on its torches and black mist in the air to really drive home the ambience
The Forsaken appears as a tall figure with a wooden mask. His body would be hazy, implying that he is becoming a spirit. He would wield a massive purple club made from wood and strange organic matter, appropriately named the “Abomination” , as well as wearing black titanium armor with a purple stash.
Once he is encountered, he will have the spirits in his body fly out for a second, before he goes in for a attack.
Pic 1: His mask
Credit to SephirothArt for the amazing art.
Battle
Fighting Him
P2 = Phase 2
The Forsaken has 5000-5500 hp, and does around 200-400 damage per attack. This would make him a formidable opponent, making it challenging to fight him unprepared.
During the fight, he would have about 7 different attacks. 2 of them wouldn’t be used until his second phase, which would activate after he reaches 33%.
Spirit Release: The ghost in his body fly towards the player, lashing onto them.
Low D.O.T (200)
Slowly blinds player
P2:
Mid D.O.T (275)
Instantly Blinds
Club Slam: Slams his club onto the ground, sending a shockwave out every second for 5 seconds.
Mid Damage (120 Per wave)
Slight Knockback
P2:
No change
Lingering Soul: Leaves some of the souls on the ground, causing anyone that walks onto it to be stunned in place for awhile
No damage
Blinds and Stuns for 3 seconds
P2:
Spawns 2 instead of one
Club Toss: Toss his club towards the player, spreading a small poison field on impact.
Mid Damage/D.O.T (200/100)
Gives a weak poison affect
P2:
After the clubs impact, ghost will fly out and grab anyone nearby, pulling them into the poison field.
Tangled Growth: After stabbing his club into the ground, dark wood shoots out in every direction, hurting those around.
Mid Damage (290)
No special affect
P2:
Causes bleed (90)
Phase 2: After getting to 33%, the forsaken will become a ghost like being, teleporting around the arena. He also gets a 50+ damage increase to all his previous attacks. It also unlocks his last 2 abilities.
Hollow Overgrowth: Levitating up to the top of the arena, he pours out waves of ghost below him, covering the floor of the arena
High Damage (350 every 2 seconds)
Slowly blinds every player in the arena, no matter if they are in the wave of ghost.
Tethered: Pulls in all the players near him, before letting out a violent shout, flinging them all back.
High Damage (400)
Pulls
High Knockback.
Drops
Loot
After defeating him, you can get a few unique drops
2-6 Dark Sea Essentia: 1/2
Corrupted Titanium Armor (Gives more defense, but also insanity): 1/10
Abomination: 1/15
The Forsaken’s Treasure: 1/18
Rare Spell: Tsunami: 1/20
Abomination Skills: Would have Skull Crusher, Club Slam, Club Toss (P2), Tethered, and whatever other move vetex would like to put on it. Would probably lean more towards warriors, but early moves could be used by mixed. The special moves have longer cooldowns to make up for their power. They would obviously do less damage as well.
The Forsaken’s Treasure: The Mask the Forsaken died for. Would give both clearsight and revealing while wearing it, and would give attack size as well.
Pic 2: Nothing in the photo is what exactly how it should be, I just threw together some lines. Also, please ignore the typo
Rare Spell
Tsunami
Tsunami is a spell that is focus on setting up more of a stage hazard than dealing direct damage, and it has two different styles depending on where you use it: Flood and Waterfall
Flood
While sending out your magic on the ground, it would cause it to spread out over a certain distance, depending on your attack size and the configurations you put in. It would be similar to a slower shockwave explosion, while dealing less damage than it.
After the move is done, it will cause the ground that the move touched to be covered in the magic type of the user, dealing damage over a set period of time. It would also apply status effects, but would be weak compared to normal.
Waterfall
While sending out you magic in the air, it would fall down in a pillar AOE below the player, hurting anything it hits on its way down for high explosive damage, and making a lingering AOE on the ground.
This AOE would be smaller than flood, but large enough to cover the area around the player and some more. This AOE on the ground would disappear faster than flood, but would do more damage than it. It wouldn’t apply status effects.
Details/background on your proposal
(Include attachments, examples, and possible demonstrations of your idea as well here.)
Misc.
Battle Easter eggs
New Buddy
During battle, once the boss is below 10% hp, you are able to throw a warding 5 supernova potion on the boss. This would instantly end the fight, causing you to “defeat” him by driving his ghost away. After doing this, a NPC version of him would spawn, and you would be able to get him as a quartermaster on your ship. 
Not So Friendly
If you throw a Insanity 5 potion at the boss, it would practically put him into hardcore mode. All his attacks would do 2x damage, and he would be 2x faster. He would also take 50% less damage. It would be a horrendous experience, but it would double his chances from then on, and give you a badge. Little reward for the determined folk.
Arena Easter Eggs
Dear Diary
After defeating the boss, you will be left alone in the cave due to him not respawning naturally. If you wait a while, his diary will appear somewhere in the cave, giving you more insight on his past.
Spooky Ghost 
After getting the The Forsaken’s Treasure, you will be able to put it in the pedestal in the center of the room. It wouldn’t do anything other than make the room shake alittle, then suddenly have a ghost appear from the pedestal to give the player a little spook before ejecting the mask.
Final thoughts
It would be fun to have a more interactive boss. Little easter eggs would make it more life like, and really help it feel like more of a character. Please ignore any grammar mistakes, its 1:30 rn and i’m really sleepy. Its alittle unbalanced, but they can do that if they ever add it.