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What I want is a special vestige for the evil NPCs. However, while bound, you cannot bind other vestiges. As a result, if one could bind multiple vestiges, he instead gets additional or enhanced abilities from the vestige. Most of the other stuff is described below.
The vestige is loosely based on an amalgam of the Beast and the Dragon of the book of Revelation; it is an apocalyptic creature, the abilities from binding it are mainly destructive in nature. It also has more deleterious effects on the binder's soul than most vestiges, as the Beast, whose body now lies dead in the Styx in a deep layer of the Abyss, was (and still is) a devourer of souls. Eventually, the binder's soul is eaten away completely, and his body is a vessel for the Beast.
Binding the Beast automatically results in a poor pact, so to speak. However, the binding check is still made to determine whether it is heavy or light influence (against a high DC); this check must be made once for every vestige slot available. While under light influence, the binder tends to revel in destruction and chaos, and must not attempt to avoid destruction or bloodshed. Under heavy influence, a binder's alignment is changed for the duration of the pact; A chaotic or evil creature becomes chaotic evil. Otherwise, each component of the binder's alignment is moved closer to chaotic evil. In addition, the binder must take any measures possible to cause chaos, destruction, or suffering.
The Beast's special requirement cannot be circumvented in any way. The Beast's special requirement is that its seal be drawn with the blood of a Good sentient creature dead no more than three minutes.
The Beast's sign increases in scale with each vestige slot occupied. Initially, it causes its seal to appear in red (not glowing) on the binder's forehead or right hand, at his choosing. In addition, his irises become blood red. Occupying two slots, the seal glows faintly, and the binder's skin gains a faint red tinge. With three slots occupied, the seal glows a bright red, giving off light as a candle. The binder's eyes are replaced by glowing red spheres, and his skin becomes a reddish hide. While the Beast occupies four slots, the binder's hide becomes scaly and his face gains draconic features. These are all in addition to any natural attacks the binder gets as a result of the vestige. ((And it's a dragon-demon-beast. I expect natural attacks.))