I love Ultrakill. While its mechanics are also extremely interesting, most of them either draw from other games or are inspired in some way.
What I want ot talk about is the story aspects of Ultrakill.
The blend of Christian and Greek mythologies is really cool and interesting, while also introducing an alternate universe where WWI never ended.
My two favorite unique additions to Ultrakill are in the form of V2 and Sisyphus Prime.
V2 introduces a supposedly superior version of yourself (V1) and makes you fight. I think thats a cool and pretty original concept.
Sisyphus Prime, while not as original as V2, still has a really cool lead-up and introduction that I think will be difficult to top for some of the coolest boss introductions in games.
On the off chance that there are other Ultrakill fans, I'd like to hear what you think.
