Grand Corpsewalker is a Secret Solo Boss in MOW MY LAWN: TWEAKED
She can be fought by accessing a secret cavern in the Abyssal Garden, after that you jump into the giant abyss.
Defeat the lowest form of the archdemon in a futile attempt to stop what is to come.
In the depths, the Corpsewalker lurked. The shining crimson light in the abyss guided the archdemon down into destruction, where their souls would duel for ultimate 'possession' over the demon's body.
Soon the deity would know sorrow, and guilt, and pain. But yet, she would know joy, and mirth, and love. The conspiracy of the three cosmic powers would be 'one third' lost.
The Corpsewalker would turn to the code of knighthood in the darkest days of the universe, and assist in bringing light back to existence. - Corpsewalker's Badge Description, Knight Errant
Corpsewalker is a secret optional boss in the game she has 2 Phases, during the first phase Her title is "Corpsewalker - Beast Unchained" and has 250000 HP, in Her 2nd phase her title changes to "Corpsewalker - Knight Errant" and gains 500000 HP as well as new attacks, Defeating Her will give you Carthage.
For Her appearence, In the first phase, She highly resembles a Satyr, She wields The Carthage like an axe, And She has an giant eye in her chest, which has an darkened appeareance and yellow iris, There also seems to be tubes coming out of the eye.
In her 2nd Phase, nothing much changes other than She gains Super Evil Wings, and now her Carthage handle is red and she holds in by the blade.
You have to come prepared with Archspear and Last Resort, you should use Last Resort when your health is full, and if you get criticaly low in HP use archspear (or in my case, use it always when damaged until health is full.), her first phase should be pretty simple, boring too, roll if you hear any sounds and pay attention to the animation Corpsewalker makes, should be easy to go to Phase 2, where things will get hellish.
you have to be very careful on Phase 2, each attack from her can deal alot of damage, if you do get in there with success, try memorizing the attacks and timing when to roll.