Tween (GEP)

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These parasitic creatures are rarely encountered in an "unattached" state on the Border Ethereal, appearing as smoky or shadowy humanoid silhouettes in the already foggy ethereal medium. Even fellow ethereal beings have a difficult time seeing tweens. Unfortunately, the tween has no trouble at all locating hosts on the adjoining material plane.
A tween is really just a parasitic spirit (although it is not undead) that prefers an intelligent being for a host. Once an unattached tween discovers a suitable host (or if an ethereal traveler runs across an unattached tween), the tween "shadows" the host, following the host's every move. Once a tween begins this bonding behavior toward a host, it becomes vaguely visible as an extra shadow on the adjoining plane (to an ethereal host, or to a host that subsequently travels to the Ethereal, the tween appears the same). After several hours with a new host, the tween remains permanently "bonded" until the host or the tween dies-neither tween nor host are able to voluntarily sever the bond. If the host of a tween dies, the shock and grief of losing its host literally causes the tween to fission in two, birthing a new tween. Both tweens then begin looking for new hosts.
Tweens are luck. Somehow, through their mimiclike bond with their host, they are able to subtly affect probability in the host's vicinity. For instance, the tween makes it just a bit more probable that its host's sword hits rather than misses. Thus, the tween's host appears quite lucky to cutters unaware that the host is infested. But the luck conferred to a tween's host is robbed from any creature near the host, whether friend or foe. A berk with a tween parasite is thus something of a curse to potential companions, and thus usually ends up as an outcast.
If attacked on the Ethereal, the tween fights independent of its host's actions. Otherwise, it is the perfect mimic. If a tween is killed, the host must make a successful system shock roll or suffer a -2 penalty to all actions for 1d4 weeks, affected by an indefinable sense of grief.
Some graybeards suspect that tweens were once members of a planar 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. Most mutated kyleen ended up in the Astral Plane as the devetes (see PLANESCAPE MONSTROUS COMPENDIUM Appendix III), but a few found new homes on the Ethereal as the tweens. What's the connection? Both races seem to lack any kind of racial identity. Devetes mimic other races and tweens bind themselves to beings on the Prime; without those ties, both races seem to lack any goal or independent purpose. (MONSTROUS COMPENDIUM 14, Fiend Folio)
TwEEN: AC {10}; MV {as host}; HD 1; THAC0 20; #AT 1 (on Ethereal only); Dmg by weapon type; SA Luck; SZ S (3' tall); ML as host or Unsteady (7); Int very (11-12); AL N; XP 15.
SA-Luck: Whenever a die roll is called for, any being with a tween parasite receives two dice rolls instead of one and applies the better of the two rolls. Other creatures within 50' of the host must make two die rolls for any action and use the inferior result.