If Henry approaches the boulder, he'll notice that he's being drawn to what lies beyond the boulder rather than the rock itself. It will continue to shimmer a bit, similar to the wall that appears to them following the ritual.

To learn more, he might need to shift or move it, but this will require the efforts of more than one person, and possibly more than one investigative effort depending on what characters attempt and how many people are working together.

You can begin an investigation here alone or with other characters.