For it's and its, it's is a contraction of it and is, so you would use it when you describe something... or something
. An example would be it's blue. Its is possessive, meaning that you would use it when there's something possessing something. for example: its blue bone.
Another grammar mistake is the difference between there, their and they're. There is a place, like "over there". Their is possessive like it's their gold bag. They're is a contraction of they and are, like they are super mega awesome.
The their, they're, and they in the second paragraph all referred to The Behemoth.
Edit: Added capitalization, which is another touchy thing. Please try to capitalize the first letter in each sentence(though it's not the most important thing), anything that has a capital(names, places, titles, etc) and not every second letter please. that can get annoying.