Behemoths are the most difficult but most rewarding monsters to defeat in Dragon Project.
Occasionally, when roaming around the fields of Heiland, you'll receive the warning: "Behemoth Approaching!" This means that a Behemoth is nearby, and you'll be teamed up with three (3) other Hunters to defeat the monster. 
When you successfully bring down the Behemoth, you'll be rewarded with rare materials that you can use to forge stronger weapons and armor, making you more formidable to hunt even more powerful behemoths!