To be honest, i actually quite enjoyed that. So far he doesn't feel too strong and doesn't have females swarming over him, that's a good sign for fanfiction. If that's your first story, then you have some talent kid, kudos to you.
Wall of text for the IxaoEclipse:
If you're looking for feedback, be careful not to make him too strong too early, many fledgling writers make this mistake with their stories. Everyone likes reading about a badass hero destroying his enemies, but it gets boring fast, you run out of things to write about if your tenno wins vs everything and it gets cringey for the reader if not done right.
One of the better stories of this kind i read split the entire novel in two parts, the past and present of our hero. The writer alternated between a novice swordsman, still a child and a true master of his craft, beating all odds with either his blade or wise mind. If i remember correctly, the tale was divided like that because the author had problems with pacing herself and that's something i saw in your story too.
In 10 pages we jumped from a normal human to a being equal to the rest of the Tenno, and in this universe, they are truly monsters (taking on entire ships with only a blade?). There's so many interesting things to write about his training and his transformation into tenno feels really rushed. Suul's training took years according to the story, but it doesn't feel like that happened at all.
This prologue is an interesting sneak peak into how this story will develop, but you could turn it into so much more. Shorten Suul's time with Hydroid to let's say a few months, dedicate a chapter or two to his stay there and make readers caught up in the story. Character development is very important in making someone feel involved and unless there's some way to indicate the pass of time, no one won't notice it ("Years had gone by." is really not enough).
Right now, Suul has everything taken away from him, his whole world crumbles in front of his eyes and the only thing to emphasize that is 2-3 lines of text. A whole chapter focused on their escape from Mars and filled with the duo's conversations would be so much better.
Another important thing is how they met. While merging with the boy is interesting, it makes no sense. Why would hydroid do something so dangerous and uncertain instead of changing into water and flowing with the stream? Or finding a small cavity in the ground and preserving himself there? No search party would suspect a puddle of water near a river (if they knew of his abilities and found a river i doubt they would continue chase).
Rambling continues, you should find yourself a beta-reader or two. For example the moment when Suul steps out of the elevator, it took me some time to understand that there are actually 4 people in the room (two grineer stood as guards and he starts describing two men, that part was really confusing to me, maybe i'm just a derp).
Tenno Hagane presenting himself to all others was a nice idea, but it should be timed better. Lotus notices Loki talking to Hydroid and immediately introduces him? Again, i think you should give such important events more backstory. You could describe that Tenno are whispering about noticing something weird on Grineer missions or maybe one squad actually saw him (he noticed them, bowed and disappeared?).
To sum all this up, it feels like you really want to write about Suul's later chapters ASAP and are omitting important parts of his development as a result (taking his grand revenge in the prologue?). It would only serve the story well if you described his tale without any details in the beginning, it would fit very nicely into the whole "formal introduction" to the other tenno.
All things said and considered, these are only advice I gathered from working with other aspiring writers starting on fanfiction.com and so on. How you lead the story is all up to you, finding your style is most important after all.
TL;DR Writing a good story is actually really hard, lol.