Always render behind or above a particular object

How to easily arrange an object like a plane of glow effect to always render behind or above a particular object, e.g. a weapon?

 

Move your objects out of the visible play area, then set up multiple cameras using the camera Depth values to control the order in which they render. For example, a -20 Depth will render before (below) a 10 Depth camera. Remember to set the Clear Flags to Depth as well. Layers and Culling are also useful here to control what each camera (and lights) can see/affect.

 

Thank you for your reply^^
Setting up different cameras with different depth works but I wonder is this the approach that normally those 3D online games do when a character is holding a magic staff with a glow effect which just always renders behind that staff? How do people manage the rendering order when they just want ONE object renders behind/above another ONE object in a 3D game?

 

I'm no expert, and I think you could achieve a similar effect by overlaying 2D sprites or something similar, but I really think the camera approach is the easiest. Perhaps most importantly, multiple cameras lets you use 3D assets (weapons, forearm and hand, animations, etc).