The message inscribed on the One Ring. What does the final line mean?

The final line of the message reads:

One ring to bring them all and in the darkness bind them

What exactly does this mean?

By them, I'm assuming it's refferring to the humans, elves and dwarves Sauron gave rings of power.

By ...and in the darkness bind them, does he mean to bind them to his will like slaves, or to trap them in some way / place? Were the elves and dwarves supposed to end up ringwraiths like the humans?

What is the darkness?

Maybe no explanation is ever given, I'm just curious. For a long time I thought it read and in the darkness BITE them. Which I assumed meant to kill them.

Could be related to this question:

In the inscription of the One Ring, what or who does “them all” refer to?

But I don't think it's a duplicate.