The material below is Open Game Content from a 3PP Source
Sword of the Planes
Aura strong evocation; CL 15th
Slot none; Price 22,315 gp; Weight 4 lbs.
This longsword has an enhancement bonus of +1 on the Material Plane, but on any Elemental Plane its enhancement bonus increases to +2. The +2 enhancement bonus also applies whenever the weapon is used against creatures native to the Elemental Plane. It operates as a +3 longsword on the Astral Plane and the Ethereal Plane, or when used against opponents native to either of those planes. On any other plane, or against any outsider, it functions as a +4 longsword.
Requirements Craft Magic Arms and Armor, plane shift; Cost 11,315 gp