Grassblades doesn't really have ONE thing that qualifies as the biggest influence - but there is a definite theme to a lot of the influences!
Here, I made one of these a while ago, to answer a similar question:
Some of these are just straight up artistic influences (Kawase Hasui, for example), but some of them are story and structural inspirations.
There is Blade of the Immortal and Lone Wolf and Cub, Okami and Usagi Yojimbo, Vagabond and Hellboy, Toshiro Mifune in Kurosawa's samurai movies (used here both because that, specifically, is relevant, and as a stand-in for all the other East Asian costume-drama movies I like, like House of Flying Daggers), and 365 Samurai and a Few Bowls of Rice by JP Kalonji - and those are just the most immediate inspirations.
Grassblades is pretty much my love-letter to all the stories I enjoy, all the story-elements I love reading, and all the visuals I love looking at, blended with a story I really, really want to tell, with themes that matter to me.