How to add weapon trail to sword swings?

I can think of two ways of doing this in Godot 3:

• Animated: You can use CSGPolygon, keyframe the polygon property in an AnimationPlayer and make sure the track is set to continuous.
• Procedural: Create an ImmediateGeometry, have it follow to points (e.g. Position3D) which can be attached to the character via BoneAttachment or via RemoteTransform, and each frame it records the positions in an array, an outputs new geometry. See Using ImmediateGeometry.

I had a look at the asset library, and the solution we can find there uses ImmediateGeometry.

In either case you need to use a material that completes the effect with color and transparency.

Note for Godot 4 users: ImmediateGeometry was removed in favor or ImmediateMesh. So instead of a ImmediateGeometry node, you have a MeshInstance3D node with its mesh set to a ImmediateMesh.