Began writing again: Death of the Digital World.

[DragonMasterX]

I think the word you're looking for to describe an alternate dimension is "Plane". Plain sounds like you were approaching to describe the relieve of some landscape instead of describing a different world as a whole unique extrapolation.

"to expand their living" sounds sort of off to me. I'd replace it with something more formal like: "to increase their life expectancy.", since you're talking about it as if it was some sort of documentary.

After their introduction to the digital plain, the Digimon begin breeding with one another and soon Nurseries of children Digimon began cropping up all over the digital world.

Woah woah woah. Wait a sec. Choose one tense to narrate in and stay with it, jumping from one tense to another can lead to a reader's premature disorientation, you can easily change from past to present, but not backwards, and much less so spontaneously.

"began to become" sounds kind of odd to me. Might wanna switch either of those words for a synonim, like "started" for "began" or "turn into" for "become".

Coincidentally Digimon began to adopt these ways themselves, copying their creators despite being innocently unaware of human existence.

Coincidences are said to have had no cause, only an effect. If Digimon are 'copying' their creators (Even if unaware), then they are the reason of their new behavior. Your sentence sounds highly contradictive as a result. I think the word you should've used at the beggining (And as a link to the above paragraph) shoulda been: "Consequently".

Resultantly, this has led to the decreasing population

We don't know and you haven't mentioned anything about a decreasing population in the entire narration (The title doesn't count). Therefore, using a definate article will annihilate your chances to captivate a reader's attention. You should remove "the" and replace it for an "a", as it's an indefinate article suited to a sentence speaking about a new or "unexpected" event.

The Nurseries were burned to the ground, the children Digimon were mercilessly slaughtered and their carers met a similar fate.

...And? Similar in what way? They were killed too? Used as batteries? Smashed and made into egg omelletes? You describe slaughter in the preceding sentence and you seem to want to finish making the point things are currently being horrible, but why did you end it so abruptly to switch onto a new paragraph? "...met a similar fate."

Hmm, other than all that above it was a pretty narration.

Onto plot analysis though, let's see... First two paragraphs is you reiterating what any basic digimon fan knows. Just having watched a few episodes of Digimon Tamers would give anyone enough basis to understand it.

Now, third, fourth and fifth paragraph. You twisted the normal thread of Digimon franchise and went for your own alternate universe, that's cool. You made humans warring assholes, also cool, kind of realist. Children became miniature and a physically weaker adults, kind of far fetched, but with a bit more of tweaking for their standard psychological revamp, it could work.

Digimon went to war because humans did. That's a nice cause & effect conflict to set the story on.

It's not a bad setting, but not good either. Probably because you just focused on things getting burned to ashes because humans went to war instead of focusing on a more in-depth cause.
Then again, this is just an introduction.

Is this going to be a fan-fiction or a RPG?