Near the end of Wizards of the Coast’s Dungeons and Dragons version 3.5, they produced some really epic material. They had all the various supplements already published, and so had a rich set of material to work with.
Into that realm came “Elder Evils”. This is a sourcebook I have gone back to again and again, just to read and marvel at the scale of the ideas that they presented. In a nutshell, the Dungeon Master is presented with the option or idea of introducing a campaign long enemy, whose presence would slowly become revealed, and whose manifestation, even in defeat, would alter the world of the players forever. Such a great way to end a campaign, fighting for the survival of your world, but leaving it changed so that new adventures could be started with a new batch of characters.
To get a sense of the things they had in mind, in one there is a small moonlet that is the concentrated opposition of creation, a side effect of the creation of the universe. Atropus it is called, and even in the end you don’t fight the moon itself, but an avatar. In any case, as it approaches the undead become stronger and stronger until those who die rise immediately as undead. All this time a demon has broken free from his master and is leading his army onto the world of the heroes. Did I mention that the army is an army of the undead? In another case the force is not death but life. This is not life as we would appreciate it, but it grows and shambles and perverts everything about it. This being, this ‘mother’, seeks to remake the world in her image, replacing all life with slimes and aberrations. In the final chapter she hurls herself bodily upon the planet, destroying a vast area in an effort to corrupt the world itself.
All of the various evils presented represent an excess of some sort … too much darkness, too much death, too much life, too many snakes (yes snakes), too much anger, etc. … and so creating ideas of your own is relatively straightforward; pick something, almost anything, and imagine what it would be like to have too much of it.
Too much air? The world is best by whirlwinds until everything is torn asunder. Too much spin? Days become shorter and shorter until objects are no longer being attracted to the planet and everything is hurled into the void. Too much earth? The air itself begins to thicken with dust until everything is choked with silt. Too much blood? People begin to bleed spontaneously, bodies of water die, and the world trembles as the first trembles of a massive heart begins to beat.
Simply put, it’s a wonderful idea to add to a campaign, to give it an overall arc. The key, however, is while there is one thing that is the focus, other elements must work into that, lest it’s all zombies / earth elementals / vampires / snakes (yes snakes), all the time. Figure out who would ally with that group, who wouldn’t be harmed (significantly) by the change, and who can adapt to that situation. That way you could have the party facing off against constructs one week and then having to deal with incorporeal beings the next. Variety.