My current D&D 3.5 campaign is about aberrations and I want to give my low-level characters an advantage against aberrations. (They're level 2 at the moment). One of my players is playing a fighter that wields a greatsword, so I want to add a greatsword that is a normal +1 greatsword against all monsters, but for aberrations, it will be a +3 greatsword instead. I am hoping this will allow me to pit them against stronger aberrations without killing them, but still keep the higher-level aberrations an appropriate challenge. There are similar things like this for undead and lycanthropes.
My question is, Will the greatsword I described above allow my characters to fight stronger aberrations than normal, keep those same aberrations a challenge, and still keep the game balanced?
By unbalanced, I mean when characters cream through the other monsters like a flamethrower through butter. (This happened to me before and it basically killed the challenge to anything in my campaign).
I also plan I giving my other characters in the party similar weapons that give them an advantage against aberrations, so as to not make it seem like the fighter is better than everyone else.