Fugaku has one child he hid away as a kid a son, his first conceived/ born child, older than Itachi by 21 years. He has earned worldwide fame as the second God of Shinobi.
Disclaimer: I do not own this story nor do I own any of its characters.
To my son
I'm writing this letter to you not to receive forgiveness, or even with any hope that this letter will reach you. I'm guessing that you will not know who I am, but I'd imagine that by now you know that you're of the Uchiha clan. I know it must've come as a surprise, to know of your heritage, but it seems, from what I've heard that you've harnessed your inherited powers incredibly.
Firstly, I am sorry I left you all those years ago, I have no excuse for that. I guess all I can say is that, me and your mother, we just couldn't let them find out that our first conceived child was a son. The reason being that for your sake I made the right choice, regardless of my reasoning.
Anyway I am not sending this letter to exchange pleasantries, I am sending it as a warning. Stay away from home. I assume you still live with Hachiro, at Kurama village, where I left you. It doesn't matter, either way you have to leave. I'm sorry, I don't have time to explain much, because by the time this letter reaches you, they will already know where to come for you. In short, I've failed. They know you exist, and they will not stop until you are found and captured and power you possess inside you secured for themselves. Being my first conceived child you possess the ability to transform into a colossal monster, and they want this power badly. For what, I can't explain right now. In any case way, get yourself as far away from home as you can. If you have a family, keep them as far away from you as possible. The things these people will be willing to do to get their hands on you is unreal, so anyone: friends, family, anyone, keep them anonymous, and get them to the safest place you know. The training, you received will allow you to defend yourself and your family from those who wish you harm, but I know you won't want to risk them.
I can't really say much more, just, when you've got your loved ones to safety, meet me at the ancient ruins at NakuyoCity. I know you know where that is. I don't have time to explain more now. I'll tell you everything when I see you.
I understand you probably feel I am trustworthy, but trust me, my leaving you, was the best thing I could have done for you, I don't need you to understand, but, for your own sake just please trust me this once. You can't handle this by yourself. I know you're powerful, but you don't know what you're up against. Their resources make them almost omnipresent, so trust no-one.
Good Luck Koichi
You're Father Fugaku