Sword of Clueless Slaying:
This uncharitably named weapon is often used by Cagers who have extensive dealings with planes not often visited by most Cagers. It has a +1 bonus on the Outer Planes; +2 on the Inner Planes or against creatures native to them; +3 on the Transitive Planes or against native creatures; and +4 on the Material Plane, demiplanes, or creatures native to either.
Note that the higher bonus is always used. For instance, if one were to meet an efreeti in Sigil, the sword would have a +2 bonus against it. If one were to meet a devil on Oerth, the sword would have a +4 bonus against it, and so on.
This sword has the same cost and prereqs as a sword of the planes, but the creator must be native to an Outer Plane and must craft the sword on his home plane. (Sigil counts as the Outlands.) By the same token, a standard sword of the planes must be crafted on the Material Plane by a Material native. (This prevents planars from effectively crafting a discounted +4 longsword.)
Curiously, both types of swords are fully effective against demiplanes. I guess those sods are just out of luck! 
This makes no sense in 3e and beyond since magic weapons don't lose pluses. That's a 2e rule.
If it was really a Sword of Clueless Slaying, it would do +2d6 damage against creatures of the prime.