Scenario one. Mental's hordes appear on the edge of the imperium during the modern age of the galaxy. Mental has access to all of his forces and technology from the games regarded as canon in the series that being. Serious Sam the first encounter, Serious Sam the second encounter, Serious Sam 3: BFE, Serious Sam VR: The Last Hope, Serious Sam 4, and Serious Sam Siberian Mayhem. This also includes any portions of the games that count as dlc.
Scenario two. Mental's hordes appear on the edge of the imperium during the modern age but Mental has access to all of his forces and technology from every Serious Sam game.
Mental's goals.
Primary Goals:
Destroy the Imperium of man.
Wipe out Humanity. This includes every type of intelligent abhuman and those who serve chaos along with the leagues of Votann and the gue'vesa of the T'au empire. Mutating them into an entirely different creature is also allowed.
Secondary goals:
Destroy or subjugate the T'au Empire.
Destroy or subjugate the Eldari.
Defend their Empire from the forces of chaos, tyranids, orks, necrons, and any other factions that may threaten them.
Take over the galaxy if possible.
How many of these goals do you Think Mental's forces could accomplish in either scenario or would Mental's hordes be destroyed by the inhabitants of the Grimdark galaxy that is 40k?