Fatalis Ecology

Disclaimer: This is merely a plausible explanation for the monster(s) in this article, and should not be considered canon.

In-Game Information
A legendary dragon said to have prowled these lands from the days of old. Many skilled hunters have sought to challenge it, but none ever return. A monster shrouded in mystery...

Taxonomy
One of three Fatalis brothers, Black Fatalis is arguably the weakest, as it possesses fewer aerial attacks and a less potent rage mode than his siblings. Black Fatalis is also a member of the Elder Dragon family, a group that is by definition very rare and powerful.

Habitat Range
Fatalis is infamous for its destruction of the once great Castle Schrade, and the surrounding town of Schrade. This region has become the primary habitat range of Fatalis. It will venture outward in search of food, only to later to return to the castle to sleep. It is assumed that Black Fatalis could survive in almost any region, it simply chooses not to travel far from its keep.

Ecological Niche
Fatalis is well beyond any ecological niche. In fact, the creature inhabits a human castle. One possible theory is that the Fatalis that the hunter fights is a young member of it's species and invaded the Castle Schrade seeing it as a suitable spot to grow. As it once was a human dwelling, other creatures tended to avoid it. Thus, the Fatalis was safe from any possible attacks by monsters larger then itself. When Fatalis does choose to leave the castle to feed, it will ravage the local countryside, killing everything from large herbivores to other predatory wyverns. Fatalis is one of the few creatures that will openly challenge, and kill, other large Elder dragons such as Teostras or Kushala Daoras.

Biological Adaptations
Fatalis is a powerful beast. It's slow but that means next to nothing. Its attacks have such sheer power that hunters flee should it target them. However this creature is particularly clever and it can quickly chase after hunter by hovering in the air and flying after them. The creature also has an ability it's other siblings don't have; it is capable of "snaking" across the ground very quickly to ram hunters. This move usually kills instantly. Fatalis also possess some of the strongest firebreath of all monsters. Only very strong fire resistant armor, such as high grade Rathalos, Akantor, or other Fatalis armor barely has the strength to withstand one blast, let alone two. Fatalis' shell is also remarkably strong, and while not necessarily thick it will deflect the blows from all but the strongest weapons.

Behavior
Fatalis is extremely hostile to all living creatures, especially humans. It is also fiercely territorial. Using its keen sense of smell it can easily detect when hunters have entered its domain and will actively hunt them down and make every attempt to destroy them. Ironically, Fatalis' home base at Castle Schrade provided the best defence for Hunters, as a number of its weapons are still operational. It is completely unknown how Fatalis goes about breeding, as two members of the species have never been spotted together at once. It is also unkown if Black Fatalis has any sense of comradery toward his siblings, White and Crimson. The horrors that a union of Fatalis' would inflict on Hunter's Guild territory is almost beyond imagination. Thankfully, Fatalis live far apart from one another. It is also a curiousity that Fatalis chose the now ruined Castle Schrade as its home, as it is man-made and forces the creature to leave every time it hunts. It could be that Fatalis has a sense of pride, and chooses to live at the sight its victory to show some form of dominance over the hunters that so actively try to slay the beast. This is all rampant speculation though, and it could simply be that Castle Schrade is centrally located enough that Fatalis has a wide variety of prey to chose from.