(Not a teacher, just a fan of fantasy literature.)
Sounds like a native speaker's rough draft, with one exception. I'd make these edits for clarity.
> This all is > This is all -or- All this is
"This all" sounds slightly non-standard. There are native dialects that use it, and it's not unclear. But it makes me pause for a moment.
> It has always been like this.
Uses too many words for too little meaning. Delete and re-read to see if you miss it.
> start ... stop
Slightly weak. It's better to phrase without these words, and the "when" already creates the hypothetical mood.
> taking everything we have for granted, we are stripped of all
"Everything ... all" creates a strange parallel. I start by thinking about "everything," then you add a qualifier, then you're talking about "all" but I'm not sure how broad "all" is.
Better to do this: Use "what" so I expect a relative clause from the beginning. And tie back to the same referent with "it."
> When we take what we have for granted, we lose it all.
>paradise we
Clearer with the comma between clauses.
>the paradise > this paradise
A native speaker might confuse "the" and "this" here. "This" creates better rhythm (I think) for two reasons:
1 - "the" often introduces a noun modified by a relative clause. So I hear "the paradise [which] we are shown."
2 - "this," compared to "the," tells your reader to try a little harder to identify what you're talking about. Since "this paradise" contrasts with "what we have," the extra clarity and mental stress is valuable.
> the hell > hell
A native speaker probably wouldn't make this mistake.
Thus:
> When we are not thankful for this paradise, we are shown what hell is.
> This war will be devastating, and when the end finds us, we will weep over the ruins of our world
Sounds natural to me. If the concept of "war" has been introduced already, "the war" may flow a little better. Again, it's a question of rhythm and what ideas you want to stress.
I'm not a native speaker, of course, but I try to do my best to learn English. May be I can sometimes ask you some questions connected with my works? I write fantasy in Russian and I try to translate it into English, so your help would be valuable! If you don't mind, of course.