Tellus: On Monsters

Any fantasy setting filled with opportunities for adventures need, by very design, to be filled with monsters. In particular, taking many cues from its Tabletop RPG roots, many JRPG which followed off from the success of Dragon Quest and Final Fantasy are positively teeming with an endless array of monstrosities, from the relatively harmless slime to gigantic dragons to much stranger beasts.

Great Beasts

On Tellus, the line between ordinary animal and magical beast is quite blurry. While there are ordinary creatures which are used as pets and/or livestock as well as plenty of animals in the ecosystem which do not bother the Ulm and others as soon as one get into the realm of predators and scavengers is when things get nastier. Beasts refer to all non-sapient (or low sapient) creatures irrelevant of whether or not they are plausible animals or what D&D once called Magical Beasts and now calls Monstrosities. These are creatures which, for the most part, lacks a higher soul. Taking cues from folklore, especially Japanese folklore, this is not the be all end all for these creatures. Beasts of exceptional breeding and longevity, which eat many sapient creatures, survive many battles and end up living for centuries often find themselves transformed into semi-unique or unique monsters.

Just how long this process take or how accurate it is remain a mystery, as well as what exactly create what abilities. It is, in many ways, just a 'thing that happen'. Some creatures simply grow in size (and often, for inexplicable reasons change color) while others undergo a much more intense and complete metamorphosis into a radically different body, such as ancient fish or snakes transforming into dragon-like creatures (which are not True Dragons, those are a different thing).

Fiends

The creatures classified as Fiends are born of the negative side of mind and souls. As previously established, all life and all magic is ultimately connected on some deeper spiritual level. Fiends are born of (or the actions of) imbalanced minds. Generally speaking they are often associated with what the Ulma call 'Black Magic' or chaos/change/raw power but this is not universal. Light and dark, order and chaos are two sides of the same coin after all. Souls which become entangled into material matters fester upon the material plane, manifesting as Fiends.

Once the soul (or at least, part of it) become entangled it take decades or centuries to usually manifest. Fiends, generally speaking, do not really retain any of their memories or past identities. They become more of an immaterial taint which slowly manifest as a warping of nature. In time as the corruption grows, beasts might become a form of Fiend while others form twisted bodies in dark places, away from all sight. Fiends of Black Magic are driven by passion, destruction, madness and raw emotion and needs, similar to the concept of hungry ghosts or what has been previously written about with Undead for the Urban Fantasy setting. Fiends of White Magic are much more rare but far more terrifying, born of stasis, faith, belief and the collective needs. These are much more intelligent and insidious creatures.

Come on, you knew he'd show
up here, right?

Particularly powerful individuals are also at risk of slowly degenerating into Fiends, even if such transformations are very rare, as Tellus is not one of those worlds where overwhelming spiritual or physical corruption is commonplace. That said, truly powerful individuals who delve deep into more primordial powers are at a risk of turning into Fiends. A few mad souls even seek to do so on purpose as somekind of twisted apotheosis. It is not uncommon for such corrupted individuals, when wounded and under stress, surrounded by enemies they cannot match to completely 'give in'...and transform into monsters which reflect their inner selves. This is not easily achieved nor is it common knowledge, but power-mad archmages and their ilks have been known to do it once or twice.

Yes, this exist entirely to justify why some individuals, especially final boss, can transform into horrible monsters or glowing angelic beings. It is not a common phenomena in the world, nor is it well known or documented. The required catalysts usually mean tinkering with ancient spells and artifacts or to become possessed by some elder evil but the potential is there.