Here is an example of a free modifier at the end of a sentence:
- Hercules killed the monstrous hydra, chopping off all nine of its heads.
Here, on the other hand, is an example of a sentence where the modifier is not free, which means you do not use a comma:
- Hercules saw the monstrous hydra swimming beneath the surface of Lake Lerna.
For more information about commas, see Purdue's OWL.
Visit Wikipedia for more information about Hercules and the Hydra. In this image, you can also see the crab attacking Hercules from behind, too!