I have done this sort of thing in both D&D 3.5 and in Legend.
Dungeons and Dragons 3.5
It’s very awkward; the item has to be treated as a Tiny Construct or something, and all the particulars have to be detailed. Balance there is very tricky. The closest thing to useful information, the intelligent item rules, don’t really work very well for a player-character.
We wound up with something like a Tiny Construct with a Perfect Flight speed, scaling magical properties on the weapon if they were swung (or that they could use as a “natural” weapon), similar to the soulknife’s mind blade and mind blade enhancement features, though scaling with HD and not as restricted. The character wound up with very few HP, causing him to die very suddenly and anticlimactically after being caught in the edge of a fireball that was primarily aimed at someone else. It’s not an easy thing to do.
Legend’s official rules do not cover this.
However, the homebrew Ego Check feat was made by one of the developers, and has been used in many successful games, including one I have been in. Because Legend is a little looser about a lot of details, and has more consistency between Types, you can easily just be a Small Construct and it will work fine (Constructs neither automatically get a smorgasbord of immunities nor lose out on a lot of HP).