Answer:
Scythe of darkness, Shadow’s light. Guiding eye of thirteenth sight.
Two brothers we are, great burdens we bear. All day we are bitterly pressed. Yet this I will say, we are full all the day, and empty when go to rest.
I appear in the morning. But am always there. You can never see me. Though I am everywhere. By night I am gone, though I sometimes never was. Nothing can defeat me. But I am easily gone.
What letter comes next: O T T F F S S ?
It stands upright and can be quite grand. Its secret is not hidden but right at hand. What is it?
All about, but cannot be seen, Can be captured, cannot be held, No throat, but can be heard. Who am I?