It's possible to set the level of megabeasts, semi-megabeasts, and titans to 0 during advanced world generation, leading civilizations relativley unmaurauded with more time to focus on goals such as attacking one another. One can control the probability of night creatures (including vampires) existing by setting the level of curses in a world to either a low or high number.
A megabeast, or titan's strength in world generation directly correlates to its cubic size: under this defenition, night creatures are the weakest creature to be considered a power by world generation, followed by minotaur, the other semimegabeasts, demons, and finally megabeasts themselves as well as titans. Each is more likely to be slain then the next in world generation, but are remarkably unlikely to be defeated in this version by anything short of an elephant due to the way world gen combat is calculated: making a creature out of slade and adamantium will do little to strengthen them in this regard if they're tiny, while even a creature made out of filth will more often then not win combat against its foes if its of large size (such as a forest titan).
If a world's population of creatures that hold power is sufficiently low enough, it's possible to change the age using proactive methods: an adventurer can slay high profile creatures in the world in order to bring it out of the age of myth, legends, or even heroes and into what is usually either the golden age or one of the racial ages (dwarves, civilization, ect.)