The Inazuma prologue can be unlocked in the Shadow of the Colossus (2018 remake) by completing all three Trials of the Colossus. The Trials of the Colossus are a series of three levels, taking place in different parts of the world ofShadow of the Colossus. Each Trial requires you to defeat a powerful colossus, solve puzzles and complete other challenges. When all three trials are completed, you will unlock the Inazuma prologue.
The first of the three Trials is located in the Forbidden Lands, on the northern border. You will need to find the Shrine of Worship and the three Cloaked Figures in order to access the Trial. Once the Trial is completed, you will receive a silver reward.
The second Trial is located in the Lands of the Colossus, at the edge of a lake. Once here, you will need to find and defeat the Two-Headed Colossus. This Trial also rewards you with a silver.
Finally, the last Trial is found on the western edge of the Forbidden Lands. You will need to locate and defeat the Great Shadow Colossus. Just like the other two Trials, this one also rewards you with a silver.
Once you have completed all three Trials and have received all three silvers, the Inazuma prologue will be unlocked. Good luck and have fun!