D20:Ethereal Plane Planes

The Ethereal Plane is coexistent with the Material Plane and often other planes as well. The Material Plane itself is visible from the Ethereal Plane, but it appears muted and indistinct, its colors blurring into each other and its edges turning fuzzy.
While it is possible to see into the Material Plane from the Ethereal Plane, the Ethereal Plane is usually invisible to those on the Material Plane. Normally, creatures on the Ethereal Plane cannot attack creatures on the Material Plane, and vice versa. A traveler on the Ethereal Plane is invisible, incorporeal, and utterly silent to someone on the Material Plane.
The Ethereal Plane is mostly empty of structures and impediments. However, the plane has its own inhabitants. Some of these are other ethereal travelers, but the ghosts found here pose a particular peril to those who walk the fog.
It has the following traits.

• No gravity.
  
• Alterable morphic. The plane contains little to alter, however.
  
• Mildly neutral-aligned.
  
• Normal magic. Spells function normally on the Ethereal Plane, though they do not cross into the Material Plane.
  The only exceptions are spells and spell-like abilities that have the force descriptor and abjuration spells that affect ethereal beings. Spellcasters on the Material Plane must have some way to detect foes on the Ethereal Plane before targeting them with force-based spells, of course. While it?s possible to hit ethereal enemies with a force spell cast on the Material Plane, the reverse isn?t possible. No magical attacks cross from the Ethereal Plane to the Material Plane, including force attacks.
  

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System Reference Document