Animate Model With Rig

How would you animate a model with the rig; not welding the basepart to the arm so the model moves with the limbs of the rig, but make the model animatable as well.

Example of what I want to achieve:
https://gyazo.com/3021d41e2c2b8b9b1d1c2ec998d563b9

As you can see, the sword moves as well as the arm producing a clean and smooth swing motion.

Example of what I don’t want:
https://gyazo.com/056050b3f014f2808a8a4e8389663c2a

The sword stays in the same position the whole animation.

I haven’t found a tutorial that includes models with multiple parts, only single meshes.
Keep in mind, the model is made of multiple parts.
Katana

If anything is unclear let me know