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A tween is a parasitic creature that needs an intelligent being for a host. They live in the Border Ethereal and scour the neighboring Prime Material Plane for potential hosts. They resemble the shadowy, smoky silhouettes of small humanoid creatures.

Some graybeards suspect that tweens were once members of a race called the kyleen, who dwelled on the Outlands millennia ago. The kyleen were infected with a strange chaos-plague that transformed the whole race. While most mutated kyleen ended up as devetes wandering the Astral Plane, some found a new home on the Ethereal Plane as the tweens. Both races seem to lack any goal or independent purpose, and either mimic other beings (devetes) or bind themselves to them (tweens).

Sources: A Guide to the Ethereal Plane, Monstrous Compendium Fiend Folio Appendix, Denizens of the Transitive Planes

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