Darker Elements by Jesse Wilson

Ten years after a team of chosen completed their last mission, the world is in a bad state all over. They haven't spoken in years and have no purpose in the world.

After a a chance encounter with a bunch of thugs, William decides enough is enough and plans to get even using Gabon, the ring of fire. This sets off a signal that attracts the others to go investigate, and also wakes up an old lab in Las Vegas that unleashes their last challenge to track them down.

One last time, the team reluctantly decides to join forces to take on the challenge, only to discover the problem was a whole lot worse than they thought. Now, five have-been heroes will need fight to save a dying world... and rediscover themselves in the process.