The Kanohi Iden was the Mask of Spirit. It allowed its user to release one's spirit from one's physical body for a limited time. While free, the spirit could move at high speed, pass through physical objects, and observe events in the physical world. The spirit was invisible to anyone not using a Kanohi Elda or Kanohi Rode. Likewise, the spirit could not interact with any physical object or communicate with anyone not using one of those two masks.
The Kanohi Iden was the Mask of Spirit. It allowed its wearer to release his astral form, which could pass through solid objects and fly at great speed, but was invisible to others.
Beings who possessed telepathy could still use it while in spirit form, since telepathy was a mental ability.
Iden users could not use elemental powers in spirit form.
The Iden could be used to scout ahead or search for beings without being seen by enemies.
The spirit form of an Iden user could not pick up objects.
Iden users couldn't touch physical things in spirit form.
The Iden was likely made using disks other than those found in Metru Nui.
It's likely that there was a Toa who wore an Iden before Matoro because Matoro got a Matoran Iden that must have been carved in tribute to the Great Mask version.
An Iden user's spirit could only be outside their body for a limited span of time. If time accelerated for the spirit, which Voporak's field might be capable of, it would cease to exist.
An Iden user wouldn't feel anything done to their physical body while in spirit form.
An Iden user's physical body showed subtle life signs when the mask was in use.
If an Iden user's mask was removed while in spirit form, the spirit would automatically return to their body.
If an Iden user's physical body was killed while the mask is in use, their spirit would die as well.
As long as an Iden user had the mask on their physical body, their spirit had access to its power. This was true even if the spirit possessed a different body; the original body still wore the mask.
An Iden user could possess another being if that being was living and had no spirit at the time.
An Iden user could only possess a body if it had no spirit and was either living or mechanical.
An Iden user couldn't possess another being if they had a spirit because a body could not have two spirits in it.
It would be reasonable to assume that two Iden users could switch bodies.
If two Iden users activated their masks, they could switch bodies.
An Iden spirit could possess an entirely mechanical being.
If an Iden user's spirit was in a mechanical being that was being destroyed, the spirit would be destroyed unless it exited prior to the mechanical being's destruction.
An Iden user couldn't possess inanimate objects.
A Suletu could not detect mental activity from an Iden user's projection.
An Iden user's projection would appear to be a ghost to an Elda or Rode user.
Rode users could see the spirit forms of Iden users because being invisible was a form of lying, as it concealed the truth.
An Iden user could communicate with an Elda user while in spirit form.
An Elda user could communicate with the spirit form of an Iden user and a Rode user might be able to. A Suletu user could not because the spirit wouldn't have a physical "mind".
- The Iden was probably Greg Farshtey's favorite Inika mask.