As a souls veteran that has played through every game in the Soulsborne etc. series from Demon's souls onwards, in my opinion there is exactly one boss that's truly unfair, Malenia, Blade of Miquella. And I do mean unfair in the technical sense, she has so may mechanics that are not fair for the player to go up against. She reacts to the players actions like no other, can switch up her behaviour at any moment and can even decide to delay some state-specific actions, making her a very unpredictable fight where the player always has to stay on their toes. This coupled with hyper-armour on certain moves, healing from every hit she gets on the player, even when blocked and that heckin' Waterfowl dance that can annihilate you in one go if you dodge poorly makes her the biggest challenge in any souls game. It's still a wonderful fight, and the dynamic reactions make Malenia feel more alive than other bosses, but after the players victory, instead of feeling like you conquered a tough opponent through skill it always feels more like surviving an ordeal thanks to more than a bit of luck.

Soo... it's safe to say that Malenia is the cheesiest boss in any game, and after beating her on four different playthroughs I wanted to get back at her, and what better way than to depict the cheesy "Blade of Miquella" as a literal cheesy blade of Miquella, or Mousequella~
(For this artwork I am assuming that the character in the promotion for the DLC Shadow of the Erdtree is indeed Miquella)

character Miquella is from From Software's Elden Ring
art by me:  Faust8593