I like the idea overall (the fragments of power redistributing to other BBEGs). It could certainly also be part of the lore/legend of the creatures (assuming that there are things known or rumored about these big bads) that is either figured out by the PC's as they adventure, available from Sages or those versed in history, rumor, or maybe the adventurers all know about this connection and source of the BBEG power.Like I said, this is a very loose idea right now and it needs some work. I’m interested in what folks think of it, how I could best implement it mechanically. I am also interested in what other folks do to try and account for PC leveling in a non-linear scenario.
And just to be clear, one thing I am not interested in is cries of “the world shouldn’t change to match the PCs!” That is a fine point of view, and is something I also believe when running certain kinds of games. But in this specific instance, I do not want that to be the case. Thanks.
I think it would also be interesting if the BBEGs are all different levels/power to begin, and then the level ups (or whatever the mechanical effect is) would then be somewhat asymmetrical, as the PCs could only make guesses as to who is more powerful than whom. Or again, another opportunity for RP or lore seeking to try to get an idea.
The actual mechanical effect is obviously going to depend a great deal on your system:
For 5e, increasing HD, damage, spell ability, Feats, other abilities, Lair powers, or other buffs as they build toward Epic-style tiers of monster would all be appropriate and could be crafted to be unique to each BBEG.
I mostly play OSE/Beyond the Wall, so skills, spell abilities, more HD, potentially resistances/immunities, and magic items and things would all be on my plate to upgrade enemies. Or even more minions as monsters and humanoids and even other humans are drawn to the growth in power of the BBEG. Or maybe even projecting that power outward as they grow - trying to thwart the PCs efforts, slow them down, buy time for that next ritual, or even direct them to a BBEG that the first BBEG has been feuding with. (all of these would also work in 5e as well).
Since I run the older style games, all of my enemies, dungeons, random encounters are divorced from the PCs levels. If they go into the Caverns of Death and Dismemberment, after being told that many other adventuring parties have never returned, that's on them. Likewise, if they are 5th level and hire themselves out to clear the Inn of rats, that's just what they're getting. PCs learn about the BBEGs, hear of their power, maybe experience it and retreat, research them, and then make a decision as to whether they're tough enough to go back in after it.