Approaching the Shrine

Before going to visit the wizard for more information about the necklace that was being safeguarded, Everett advises you to make your way down the hill, across the river and up the far side to the location of the shrine of the cultists. They have been watching the shrine for activity without noticing any until the last week. They decided to conceal the artifact and scout the location to decide what was causing the increase in activity. Before they got their plan of action finalized, they were attacked and the outpost destroyed. Everett has no idea what happened to any of the other watchers. There is a chance that some of them were taken prisoner by the cultists, another reason to go there and assess the situation.
The trail down to the river is clear. The trail itself turns before reaching it to run out to the moor. There is a path through the forest to a small inlet. Tucked into the underbrush nearby is a small boat. Crossing the river presents a minor challenge. The boat can carry three people at a time, and someone with proficiency handling water vehicles can steer it across the river with a successful DC12 Athletics check. Without proficiency, they need a DC16 success roll.
On the far side of the river, there is no path available. Searching for signs of passage reveals a concealed path up the hillside several hundred feet further upstream. DC14 Wilderness check will reveal the recent passage of two dozen kobolds. With a DC18 check, you notice several human footprints mixed in among the others. It is impossible to determine how many people passed this way, but clearly there were more than two.
The trail is clear enough to follow up the hill. After a mile, the forest opens slightly revealing an open hillside scattered with scree and loose boulders. The path you have been following continues up across the open hillside toward an overhang above that conceals a dark opening beneath it. Passive Perception DC15 or active DC10 reveals three guards posted on the hillside, two above the opening, and one about halfway up the slope away to your right behind a boulder. None of them have noticed you just inside the protection of the forest.
If you start across the open hillside, one of the two above will drop down and enter the cave. The archer on the right will wait for you all to fully expose yourselves on the hillside before firing at you. If the party notices the posted guards, they can skirt across the edge of the forest staying concealed with Stealth checks opposed by the guards passive Perception. There doesn't appear to be a way to cross the open hillside without alerting the guards to your presence, but while concealed, the party can plan ways to deal with them in order to prevent their alerting the cultists within.
The guards will try to remain hidden until they can assess the danger level, then they will send a runner into the cave to alert the others. This gives at least one round, probably two before they leave their posts.