In the most recent arcs we’ve learned about the advanced forms for the different types of Haki, namely Advanced Observation Haki (Future Sight) and Advanced Armament Haki (Emission).
However, so far we’ve not been introduced to an advanced version of the third type of Haki, Conqueror’s Haki.
So far, its most notorious usage has been for knocking out weak characters and taming wild creatures. Though still useful in the right circumstances, it’s safe to say the practical application of this type of Haki must go beyond what we’ve seen so far in the story, or it wouldn’t be such a terrifying power, as it’s been mentioned multiple times.
What this theory proposes, is that there’s one potential use of Advanced Conqueror’s Haki we’ve already seen in the story, and that is the power to summon, and even control, storms. The range of this ability can go from summoning storms (depicted as the clouds turning black), controling the wind, the waves, and even lighting. The idea behind Conqueror’s Haki seems to be about establishing dominance. So far, that dominance has been over other people and wild creatures. That’s why an evolved version of this ability would have to take it one step further, establashing dominance not only over others, but over nature itself. That’s why this would be Advanced Conqueror’s Haki.
We’ve seen situations in which effects of this nature have been present in the story without explanation, and though some might claim they’re just added for dramatic effect, it’s worth going over all of them, as many of them involve characters with confirmed Conqueror’s Haki.
In Chapter 100, Dragon saves Luffy from Smoker by summoning a gust of wind. Although as of this moment, we don’t have confirmation of Dragon’s powers, it’s a safe bet that he has Conqueror’s Haki, and probably a very strong one. Also, assuming he was the one that saved Luffy from Buggy the previous chapter by summoning thunder, and what Smoker says a bit later about how the wind is blowing in a direction that’s helping the Going Merry escape, we can assume he has a control over the storm, and it isn’t just a natural fenomena that follows him around.
There’s more evidence of this in Chapter 586, when Dragon meets Sabo. We also see there’s a storm in this scene. Later, when the Revolutionaries help the people of the Grey Terminal escape from the fire, the wind creates a path by putting out some of the flames. It’s still possible that Dragon’s power might be connected to some sort of Devil Fruit rather than Haki, but we’ve also seen some similar effects around other characters.
Another character who’s often followed around by storms is Roger. In Chapter 0, when Roger and Shiki face off against each other it’s in the middle of a storm, and the sea is raging. Though we don’t see the fight, we know that the storm sunk half of Shiki’s fleet, yet it seems like Roger’s crew was unharmed. Furthermore, in Roger’s execution, as soon as he was killed it started to rain. Though this might be unrelated to this theory, it could also be explained as Roger stopping the rain with his Haki, and then as soon as he passed away, that pressure was released and the rain started to fall.