Tween

Tweens were parasitic spirits endemic to the Ethereal plane, though they were not a type of undead.

Description
When not attached to any host, tweens appeared on the Border Ethereal to those on the Prime Material plane as smoky or shadowy humanoid silhouettes. Typically this was reminiscent of a squat human.

Personality
Tweens were very intelligent spirits, but they lacked goals or independent purposes.

Abilities
Tweens possessed a limited form of shapechange and could only be harmed by creatures who could enter the Ethereal plane or had some magical effect like oil of etherealness. While attached to a host, they could move material things at short distances.

Bonding
All tweens sought out creatures on either the Prime Material or ethereal plane to bond with, having a preference for intelligent ones. They often took hosts that were humans or demihumans.

Once a suitable host was found, tweens would shadow their target's every move. When this bonding action truly began, it would appear as a vaguely visible shadow adjacent to their host's own. After several hours, the bonding became permanent, with neither the tween nor their host being capable of voluntarily severing it. If a tween's host were to die, it would cause such grief and shock for the spirit that it would fission into two tweens.

Probability
Tweens had the ability to subtly alter probability in the vicinity of their host, which extended out to roughly, and they could see a few seconds into the future. Through these combined abilities a tween granted their host greater luck. However, this positive probability was robbed from others within their host's vicinity, whether they be enemies or allies, afflicting them with bad luck.

Combat
Tweens fought independently of their hosts when attacked on the Ethereal plane. They typically fought with some ethereal form of weapon, like a sword. Otherwise, they mimicked the actions of their hosts in combat.

History
Some scholars suspected that tweens were once members of a race that inhabited the Outlands known as the kyleen, who had fled to the ethereal plane after a strange chaos plague transformed them.

In 1357 DR, tweens were known to be lurking around the Shadowdale region.

Languages
Tweens were capable of communicating telepathically with their host.

Relationships
Creatures bonded with tweens were considered a mixed blessing by those around them. This often led to the host becoming an outcast.

Trivia

 * Zunzalor's harp could make a tween clearly visible, outlined by blue radiance.