Is it
An ocean ?
Whether I am feared or loved,
I continue to grow in power,
Once together a mighty force,
I care not for things that tower.
You can love /fear an ocean, once all the water mass was united and water's force is unprecedented.
Supposedly an equal to another,
Yet he claims domain over me,
A body that appears just as vast,
For this is an illusion that I see.
Water is supposed to equal to wind (in properties and power), however wind claims domain(atmosphere ) over water, on the Earth.
Those who claim to greatness,
Are but fools who provided none,
Bring you worldly treasures to me,
For they all get swallowed as one.
During Tsunamis and deluges, all that was saved by different people goes and submerges into depths of oceania/ water bodies.
My power is locked at two ends,
But I constantly free it some more,
Eventually I will control all that is,
For all but me is only an eye sore.
A sea/ An ocean has two ends (widely speaking, two shores) but it is available at across nations/continents. Next part needs to be explained.
The mysteries of me remain hidden,
My beauty shape is unknown,
Many have tried to discover truth,
But they fail as secrets are my own.
Mysteries of oceans are known as legends/mystic tales today across the world across generations. Many navigators/explorers tried to discover the depths of the oceans but still there remains unexplored secrets.
Those which are strong fight me,
But they forsake their own weak,
I will fight all at once for control,
They never get the victory they seek.
Those who can row / use power engines can fight against the tides of an ocean but unarmed/unequipped if we encounter ocean/water-body waves, we are always losers.