It starts with murdering everyone in your home village, including a young mother and her husband, though Danette and Lady Lanya survive. After every battle, a not-being-redeemed-in-the-slightest Gig possesses your body for some multiple of eleven days, likely increasing the bloodshed.
As the story progresses, everyone you forcibly conscript into your army goes insane, with the chivalrous Thorndyke becoming a Blood Knight considered one of the saddest changes.
When you finally meet Trish, she's a Broken Bird, implied to be because Hawthorne already raped her by then. And she dies a few chapters later, breaking her sister's spirit completely.
Laharl:Do you have any idea how hard it was for me to get that title?
By the end of it all, Lobo even refuses to work for you any longer, almost every other villain in the game has joined forces with the surviving protagonists to take you down, and winning the final battle leads to you devouring Gig's soul and completely annihilating Haephnes, Drazil, and yourself. And Revya's motivation for doing all this? Probably the same reason the player picked this path: because it seemed fun.