Rewards: 3.5kk exp, 25 task credit, and the King's Relic.
Few quick notes:
King's Relic can be held in your ammo slot, gives +Magic Find, and is enchantable. It can be enchanted as many times as you want!!
Shoutout to the following for their help:
Go into the rebirth area, and take the ladder going downstairs of the depot to find [Quest] Explorer Ryan and talk to him.
After talking to him, go downstairs and kill any of the monsters to fill up your "vial" with slime.
Warning! A knight is highly recommended to block.
Once your vial is at 100%, Report back to Explorer Ryan and talk to him once again.
After waiting 1 hour, return back to Explorer Ryan for your next mission.
Go back downstairs and follow the path in the pictures below to find the King of Slimes. It will be best to bring as big of a team as you can gather as it has a lot of health, and it hurts. (I forgot to take a screenshot of the fight with him, I'll upload one later if I find another team going through the quest to kill him.)
The star is where you start off on that floor. Head to the down arrow.
Once there, step on the black tile in the middle of the room. Make sure the knight is ready!
After defeating him, return back to Explorer Ryan once again.
Now, head to the blue flag on the map to kill the final boss. This boss you must kill solo, so make sure you have plenty of potions. Mages it would be a good idea to be level 250+ with a decent hp set. I was combo'd at level 255 with +100hp bonus. :/
Say a quick prayer, and enter the portal to face the boss. He will summon 10 towers over a small period of time that throw a small poison aoe. It's best to stand diagonal of him, and when he summons a statue, move a few steps to be out of the tower's aoe. Repeat until he's summoned all 10 and then stand in the open. After killing him, enter the door and through the portal to the rewards room. Collect the 3 chests and then you've successfully finished the quest!