Build your offline maps for Windows Phone 8.1 with MBTiles.
var uri = new Uri("ms-appx:https:///Maps/geography-class.mbtiles");
var file = await StorageFile.GetFileFromApplicationUriAsync(uri);
MapControl.TileSources.Add(MBTileSource.Create(file.Path));
See GeographyClass sample project for more details.
MBTiles are SQLite databases. Therefore, SQLitePCL package must be installed in a target project.