Riddles

[WarBringer]

Bringer of hope and of despair
Slays they mightiest and spares the weak
controlled by the Might's Children
Shapes land,
surrounds islands...
what is the answer?

Bringer of hope and despair-those who travel on sea have hope, while those caught in storms and such despair,
it can slay the mightiest, and then spare the weakest(though by chance or by design)
controlled by the Valar
it shapes land, and it surrounds islands