Yes and no...
That is to say, I will often create rules or adapt rules/items/features from another source in order to provide an encounter or adventure with a unique condition. For instance, I wanted to establish an adventure where a sub-villain (read: occupant of a mountain fortress under which lied a forgotten temple where in the true villain dwelled) created monstrosities out of several of his followers in order to better serve and protect him. The Core Rules didn't provide such, but I found a Base Class in AEG's
Magic (Flesh Mage) that fit the bill near-perfectly.
For background, I came up with the idea that the fortress had been built by a crazed Necromancer. Having died many centuries ago, his Norker slaves were now without a master. Most of the Necromancer's lore was lost, damaged, destroyed, etc., but some of it survived. Eventually, one Norker, somewhat brighter than the others, managed to decipher the writtings that remained and began to learn Arcane Secrets. In time, this new Base Class was "born" within the mountain fortress as the lore was passed down from generation to generation.
As most of the lore was passed on by oral tradition (class tweaked to be more akin to Sorcery than Wizardry), and as the Norker Flesh Mage and his apprentices were most likely going to get killed, this was a virtually guaranteed unique condition that would never appear again and, most likely, would not be available to the PCs
unless one of the PCs made it a goal to salvage the scraps of written lore and rebuild the "science" of flesh magic. There was a chance to capture an apprentice or two, which would help preserve the lore. And even capturing the Flesh Mage himself (whom is relatively weak when denied his augmented servants) was possible.
The over-all effect is actually quite nice: A flavor-element that is intended to be unique but, through the actions/choices of the PCs, could become more common if pursued, studied, researched, and preserved.
Now, there
are times when I stretch the bounds of the rules a tad. While I don't use Epic Magic all that often, it is a "fact" of the campaign world that such has existed before and will likely exist again (possibly via the PCs). In this, when
fitting, I'll introduce an area/effect quality or item that can't be quantified by the standard rules. I don't bother to determine how it was done "epicly", although the fact is, Epic Magic does pretty much allow
anything to be done. Granted, I wouldn't do this if the PCs actually encountered an Epic Spellcaster, as the specific abilities of such an NPC would require quantification in order to be used correctly (be the NPC an ally, foe, or plot device). But when found in ancient ruins, lost towers, forbidden temples, and the like, I see no need to spend to much time on it because Epic Spellcasting simply makes it possible.
As with anything else, the key is not to screw the PCs over; if used as an "oddity" or some such, great, but GMs should be careful to avoid railroading or (just as severe) bottle-necking the choices available to the PCs. For example, a painting (such as on the ceiling of a cathedral) may be empowered with Epic magics to depict the eternal war between Law and Chaos. While the painting does not move quickly (i.e., you can see movement only by staring at it for
hours, with that movement being ever-so-slight), it does accurately represent the cosmic balance (the more Chaos reigns, the more the servants/champions of Chaos occupies the painting). As this has no bearing
at all on the PCs, there's no need to quantify it. It serves as a plot device (the PCs may gleam information from it through careful study regarding the War Eternal) and as a symbol of the potential power available within the campaign (the ever mysterious "dweomers" of ancient eras that a PC may rediscover and eventually master).
So, yeah... My answer is "yes and no".
