[screen_name]

How about...

When you log in and if you have played Graal Kingdoms, then it would load your character into the forums and you can fight that way, but it wouldn't affect the server.

You can challenge people, but they must accept.
You can request to cancel the challenge, again, they must accept.
You can not challenge or do anything else with your character until the challenge has ended or been cancelled.
Before the challenge begans, you choose weapons and armor or what not.
Have it turn-based, on a turn you can attack, cast a spell (drop-down list?), use items such as potions, or perhaps choose to defend (reduces damage majorily).
Keep a record of wins/losses just for fun. Maybe reward the top three players each month with something special.

You get the general idea.