It's been a long time since I've reviewed Digimon fanfiction, but since you're dealing with characters that I know very well (the Adventure 02 children), I decided to stop by. And just to pre-warn you, I use the original names for everything, so just tell me if you don't really follow what I'm talking about.
To start things off, it's fun to read Adventure/Tamers fics. I'm always up for the Tamers meeting the Chosen Children that they emulated in Digimon care and seeing how the Chosen Children react to being a merchandising franchise. It's a fun crossover, and you do have a way to get the two worlds to me.
Our good friend Armageddonmon decided to make another comeback
I'm horrible on dubbed names for Digimon. But on checking, I've found that the dub changed Armagemon to Armageddemon. Just make sure to spell canon names correctly, okay? ^^ There are a lot of strange canon names to keep straight in both the Adventure series and the Tamers series, but there's no real need to keep them all straight. Just keep your focus on the spelling.
Arc1: Part a:The power split, the balance of two worlds.
Strangely, I think that the sentence here should be like the Act 1, Part A title. You do go on to talk about the splitting and recombining of the worlds, their powers, and how the Earth helps the Digital World.
The two worlds balancing each other out, there are different universes, all having few things in common.. Digimon, and groups of children who save worlds with these Digimon, being the Digidestined.
Those two bold full stops (which should just be one at any rate) would work better as a colon. So it would be "all having few things in common
: Digimon, and groups..."
Another thing these universes have in common, in the constant threat of evil. Of darkness, a power battle between good and evil.
Second sentence is a fragment. You could combine it with the first one and have a long one sentence. Or you could combine "of darkness" with the first sentence and add a "It was" to the rest of the second sentence.
Now, in two universes of Digimon, a powerful Digimon plans to make its comeback.. Armageddonmon.
Again, another colon. And I'm having a bit of trouble thinking that Armagemon is "in" the Tamers universe. Last I knew, he was only in the Adventure universe, so you might want to spend some time telling the readers where he is now. Is he, perhaps, in some other dimension that's unconnected from the Tamers and Adventure, and can meddle in both while not being in both?
His dark power meets no stop, being defeated twice by the powers of Digimon.
The first part makes no sense. And the second part could be added onto a bit more. Describe the battles and the lengths that the Chosen Children had to go through to defeat him. Make Armagemon more of a threat to the readers by showing his power.
But slowly, since his second destruction, he has been slowly forming himself back together, gaining more power, slowly planning to make a comeback.
You should try to look back over what you have written to catch mistakes like this. "Slowly" should only be used once here. If you want to show how long Armagemon has been gathering power, say something along the lines of "He waited patiently for his strength to return to him", or something to that effect.
Only negative outcomes will emerge from this Digimons plot, crossing over in power.. To bring two Digimon universes together, destroying the most powerful Digimon to ensure the world take over of the Digital Plain and the Human world.
To form the possessive, it's
apostrophe-s to make it "Digimon's". The same holds true for the Armageddemon's in the last paragraph. Again, the bold two full stops should be a colon. I can see what you're saying with the underlined part, but it just sounds kind of clunky to me. I guess you could try to reword it to something like "the take over of both the Digital Plain and the human world". And yeah, I don't think "human" needs to be capitalized.
You did well in forming a plot. It's just that the grammar issues need to be worked on a bit to make it a bit cleaner read. Also, you should take the time to describe things more. Like I said with Armagemon's power, you could show him sitting wherever he is, thinking back on his defeats at the hands of the Chosen Children. This shows that he wants revenge, why he wants it, and just how powerful he is. (I mean, he's back after being defeated twice by two strong Ultimate* Digimon! That shows some real perseverance if he keeps coming back.)
Other than just the basics, you did well. I can't wait to see how you do with the characters. ^^
*And Ultimate is the Japanese name for the Mega stage.