Tween

In its native Ethereal Plane a tween is a stunted, homely humanoid with short limbs and no neck to speak of. Seen from the Prime Material Plane, it is only a faint smudge of smoke or shadow hovering within six feet of the creature it has chosen as its host. After a few hours with a new host it slowly takes on that host's general shape and features, so the host seems to be followed by an extra shadow.

A tween can fight or be harmed only on the Ethereal Plane, or by tricks such as coating a blade with oil of etherealness. In a fight there it uses a weapon, typically a sword. It cannot attack anything on the Prime Material Plane and rarely needs to defend itself there.

Its true power is a glimpse a few seconds ahead in time, which it whispers to its host below the level of conscious thought, combined with the ability to nudge light objects short distances quickly enough to steer a swinging blade onto target. In game terms, the host rolls twice for every die roll, whether attack, damage, saving throw or anything else, and keeps the better result. Onlookers who do not know about the parasite simply see someone uncannily lucky.

Everyone else pays for it. Any creature, ally or enemy, within 50 feet of the host also rolls twice for every die roll but must keep the worse result, because the tween feeds on the luck it drains from them. A character carrying a tween is a curse to companions, and parties that learn the truth usually drive the unlucky host away.

Other ethereal beings regard tweens as about the lowest life on their plane and avoid them, and tweens even avoid one another. Most therefore abandon their lonely existence and attach to someone on the Prime Material Plane, living through that person. They favor intelligent humans and near-humans but have no special taste for adventurers.

The bond is permanent until host or tween dies, and neither can choose to break it. When a host dies, the tween's grief is so violent that it literally tears in two, producing a new tween. The original seeks another host almost immediately, and the newborn does the same as soon as it discovers how dreary solitary ethereal life is.

Without a host, a tween eats nearly anything it comes across on the Ethereal Plane. Once bonded, its body stays ethereal and needs no normal food, living instead off the luck of those around its host. It has neither predators nor allies, which is why most newborns find a host quickly.

Sages have hunted for a way to part host and tween without killing either one. A wish is known to work. Attempts to combine dispel magic, remove curse and plane shift have so far ended disastrously. Prevention is easier: a tween is usually put off when the host wears the amulet of proof against detection and location, since the creature cannot judge the Intelligence of a potential host wearing one.