Tales of Symphonia/Devil's Arms

After finding one of the Devil's Arms, talk to Abyssion in Flanoir. He is located by the boarded up building near the entrance to Flanoir. He will tell you a story and then give you Nebilim and Nebilim's Key.

The Abyssion Battle
After getting every Devil's Arm, go to the Shadow Temple. At Shadow's seal (bottom level), you will find Abyssion. After a short cut scene, you will fight him. Hit him with everything you have and have a lot of recovery items (life bottles, lemon gel). Eventually, he will go down (he has 120,000 HP), and give you usage of the Devil's Arms.

Aftermath
After you win the Devil's Arms from Abyssion, you can utilize their full potency. Every enemy you defeat with an Arm will increase that Arm's attack by 1 point. In this manner, you can create immense damage with just one hit.