If I’m understanding you correctly, your DEM file represents terrain, and you want to show that terrain surface in Cesium. Is that correct?
Cesium doesn’t have built-in support for rendering terrain directly from a DEM file. In fact, unless the DEM file is quite small, rendering it directly is impractical. So you have a few options:
Import your DEM into an ArcGIS Server and access it in Cesium via ArcGisImageServerTerrainProvider.
Convert it to the format described here and access it via CesiumTerrainProvider.
Implement your own custom TerrainProvider to access your terrain data in whatever format you prefer.
Purchase AGI’s soon-to-be-available terrain server, which allows you to import terrain in a variety of formats by simply dragging-and-dropping, and then easily visualize that terrain in Cesium.