Common Blender issues
Open the model in Blender, make sure you are in Object mode (bottom left corner) then press Z -> Wireframe to see through the model, afterwards right click on any bone to select the armature.
Then press R to rotate, then Z (vertical axis) then the degrees to orientate the model to the correct position (i.e 180 if the model walks backwards) then left click to set that rotation.
Lastly, with the armature still selected press Ctrl + A and Rotation & Scale to set the transforms.
If your model has multiple root bones, Terasology may crash on game startup with the following error:
19:02:42.633 [main] ERROR o.t.engine.modes.StateLoading - Error while loading org.terasology.engine.modes.loadProcesses.LoadPrefabs@38394dc3
java.lang.IllegalStateException: Cannot create a skeleton with multiple root bones
To fix this, open the model in Blender, select the model's "Armature" and make sure you're in "Edit Mode". Then click on "Add" to add a new single bone. Double-click the bone in the armature list to the right to rename it to "Root". Now select one of the original multiple root bones and Ctrl-select the others as well. Last, add the new bone called "Root" to the selection with another Ctrl-select. It's important that "Root" is the bone that is selected last. Right-click in the main window to open the "Armature Context Menu" (if the "Object Context Menu" opens instead, then check again, that you're in "Edit Mode" as that's as sign you're in "Object Mode" instead). In the "Armature Context Menu", hover over "Parent" and click on "Make" and in the following selection on "Keep Offset". You should now only see your "Root" bone at the root level of the armature and all previous root ones below it.
