Hey Ravendarke -
In order to use the DBuffer Decals you will need to add the following to under [SystemSettings] either in the BaseEngine.ini (in the Engine Directory will effect all projects) or the DefaultEngine.ini (in your project’s config directory will only effect the one project):
r.EarlyZPassMovable=1
r.EarlyZPass=2
r.DBuffer=1
For the normal Deferred Decals and based on what your opacity mask, you would want to use the Translucent Setting instead of the Stain setting.
Thank You
Eric Ketchum