I remember very little from the film, unfortunately, and I didn't manage to find anything resembling it online.
I watched it around 15-20 years ago. It is possible it was a little less, and, of course, the film might be older.
I think the animation style was like the Disney films of the time (or the older Disney films): Aladdin, Black Cauldron, Sleeping Beauty, Snow White, etc. Obviously, I realize that there is a huge difference in animation between Snow White and Aladdin, but, unfortunately, I don't remember the visuals well enough to tell whether it was more like one or the other. I'm pretty sure, however, that it wasn't like Pixar films, such as Bug Life or Toy Story.
The most important thing I remember is that, at some point, the minions of the antagonist fused with their weapons. At least I'm pretty sure they were minions of a "main" antagonist, and not just the enemies proper. I remember the color green very vividly: there was some green liquid/gooey material, which I'm almost certain allowed/caused the fusion. I think there was a hole or a pit somewhere; I think the main antagonist threw his minions (probably without asking for their consent) into a pit with some gooey, green material, probably along with their weapons, and they came out as a kind of magical half-human half-weapon creatures.
I think I remember a forest. I am not sure, but I think the scene with the pit and the green goo took place inside a forest, and perhaps much of the film took place inside a forest. Again, I'm not sure.
I think there was some artifact/object that both the protagonist(s) and the antagonist(s) were after. It is possible that the green goo had something to do with this artifact, was related to it in some way; I'm not really sure.
It's been quite a few years, so, sadly, I don't remember much else. I'm not even sure I got all the details above right. For example, it is not impossible that I got totally confused over the years and that it was an animated series rather than a film.
What I'm really certain about is that it was around 15-20 years ago, that it was an animation, and that some people were fused with their weapons. I am almost certain the color green took part in it somewhere.