I've had a friend, in a 3.5 / Pathfinder game, pull up a creature with abilities I haven't seen. It (in his description) was a shade, but it had the ability to mimic my movements and upon contact, take my physical form and turn me into a shade. Any explanation?
That's the Shadow Simulacrum Template
It was introduced in Shadowdale: The Scouring of the Land (pg. 156).
The template can be applied on a living creature, it must have 10 HD or more, and a minimum Charisma score of 6.
It grants a Supernatural Ability called Possession, which works similarly Magic Jar, but with some changes. This posession has no set duration, and no way to free the target is given, other than killing the possessed target, or abilities that specifically deal with posessions.
The creature is incorporeal when not possessing another creature, and lives on the plane of shadows.
By removing a part of your soul and replacing it with the essence of shadow, a Shade could be produced. This is an extremely complex and painful process but the result is a being with a prolonged life, and a greatly strengthened set of abilities when in the presence of shadows.