To a caster, a spell of planar binding is a simple matter; choose your target, begin your casting, and if they haven't fought off the summons, they appear in your circle in a matter of minutes. But something must take that target from the planes and bring it to the caster: that something is in fact a spell crystal. These objects are the physical manifestations of the call of a spell of planar binding, taking the form of a spindle-shaped, translucent-blue crystal flying above the surface of the plane towards its selected target at extreme speed. Those with non-specific targets materialize randomly near a large conglomeration of the selected type, while those sent after a specific target materialize within a few dozen feet of them. In both cases, the crystal flies at their target at speeds easily up to 100 miles an hour, shattering on impact and, if not resisted, instantly forming a vortex to the Prime in the process, pulling the victim to the one that called them. On rare occasion, a crystal will strike an improper target on the way, and what happens as a result depends on what exactly was hit: if they are simply a mortal, or even an inanimate object, it passes through them as if they did not exist. But if they are themselves a planar being, they are effected just as though they were the original targets. (There are some exceptions, as always — if forced to travel through a large amount of solid matter, spell crystals will shatter midway through passage to no effect, for example.)
Enough planewalkers have found themselves annoyingly in the path of spell crystals — whether intended as the target or not — that a number have developed means of capturing, or even redirecting them. A few arcanists have found means of weaving the shattered remnants of a spell crystal into a net that itself is able to safely restrain the crystals when caught, either for simple protection, or to send them against an enemy you wish to be rid of for at least a time; even long after the Prime spell is cast, the spell crystal, if captured, is still active, and will still send the victim to the Prime for a period. Some have even found the crystals, when whole, to themselves serve as spell components in spells of conjuration or abjuration; when properly drained of their energies first, of course. However, because of their general danger and unpredictability, nearly every significant city across the planes has banned their sale, no one from celestial to fiend or anywhere in between much liking the possibility of being inadvertently consigned to obedience of some Prime conjurer should a crystal get loose.
No description of spell crystals would be complete without mention of the strange Outlander beast known as the demarax. This creature seems to literally feed on spell crystals, seemingly immune to their effects. Such an odd beast is unlikely to have come about by any natural means; some believe them to be creations of the deities of Law, given their far more common appearance Lawside on the Outlands, constructed and strewn through the plane to reduce the chaos of random, unexpected summonings to the Prime, while others believe them an experiment of the Guvners or modrons gone rogue. But, given their general pacifism and extreme resistance to magic, most hardly bother to think of them at all.
- Planescape Campaign Setting - A DM's Guide to the Planes, pg.43
- Planescape Monstrous Compendium Appendix II, pp.22-23
- Planewalker's Handbook, pg.138