Hello Anthony,
So from what you described is that you are attempting to carve out an area in the landscape for the use of a cave or an area where you would expect to see a skull.
It is entirely possible to delete a section of the Landscape which creates a square area in accordance with grid size you used to create your landscape. You can that add a static mesh that you have created in a 3D modeling program. Lastly, you can disguise the entrance with foliage or rocks until you come up with something that is acceptable.
At this time detailed sculpting is not a feature that is supported in UE4. Many other programs like Zbrush/Mudbox/ or another modeling program of your choice can handle this procedure much better than UE4 every could. Developers will sometimes opt to create the entire landscape outside of UE4 and then import it as a static mesh.
You certainly can sculpt the landscape inside of UE4 but it’s features are somewhat limited.
Thank you for reporting this, I hope some of this helps better inform you of the features and limitations of the Landscape.