Nil Is a Solo Boss In MOW MY LAWN: TWEAKED
He can be fought at the Grey/Black Portal, Right behind where you spawn, He will be your First Boss.
"Defeat Nil, a testament to what was.
Deities are the eternal watchers of their separate domains, and are usually named after their places of origin. They answer to the all-encompassing super deity All, and seek only to maintain the order of the cosmos.
What would happen if one of said domains were wiped out? What would happen to the keeper? Nil is the answer to these questions. Not even a record of the world Nil watched over remains, as the memories of the people and places are lost in its unsettling blank stare gazing into eternity.
Or perhaps Nils are the prototypes of a horrible experiment in mass production of divine beings, formed by a depraved god or goddess." - Nil's Badge Description, Disgusting Beast
Nil is the First boss of the game that you will fight, He has 7000 HP, The Server Size is 1, and you have 7 lives, with this and your mower, you are guaranteed to get screwed up.
Defeating Him will give you ZeroSumValue
As for His appearence, He has Black torso, one of His legs is also Black, His Left Arm is A Giant Black Spike, While His Right Arm is Mutated, Horrific, Truly Disgusting.
When you spawn in the arena, go directly to Nil and try avoiding all of his attacks if you can (Especially the Black Orb Wave), once you get to him, get into shiftlock mode, and move your mower left and right on him while jumping, Nil will take ALOT of damage from it, if you die but removed a good part of Nil's life, then you are doing a good job, continue doing this until you finish off Nil.