SO my players are about to arrive to the alter below betrayers rise and they are on bad terms with the rivals and I know that the campaign says that two things can happen either the rivals get the amulet of three prayers or the players keep it.

the problem is I'm new to dming and I don't know how to make it so that the rivals have a chance to get the amulet.

I was planning on using the optional rule of disarming so that when the rivals and try to steal form the players during combat or that one of the rivals sneaks up while they are having the vision but that seems cheeky and unreasonable since the rivals are suppose to arrive after the players see the vision.

as well as the rivals really have no way of using any range abilities to retrieve the amulet form the alter they have to walk over there.

I also don't know how long the fight is supposed to go before they give up on the amulet like do they fight till some of them are downed.

I just could use some ideas on how to operate this encounter since the campaign doesn't give many suggestions

If I remember correctly, everybody present sees the vision. 

The rivals should have been hired by one of the faction representatives that the party didn't choose. So that NPC should be travelling with them and be able to help them. Whoever is going to get the alter has to jump down and get to it. That will trigger the vision that everybody sees. Then it'd be dependent on which NPC is allied with the Rivals, but basically it should come down to a quick fight where the Rivals are targeting the Amulet Bearer. If the Rivals are getting overwhelmed the faction NPC opens the gate and they escape. 

The players might follow through that same gate, but what happens next will depend on which faction opened it.