I have old debt and a legal department is threating to serve me papers and sue.

In your state a debt collector/creditor has only 3 years in which to sue on a credit card debt.  However, this is 3 years from the last activity on the account.  If after the charge-off you still admitted to even owning the debt a new time period may have restarted (this is called the "statute of limitations").  Assuming that is not the case, this agency is time barred from filing a suit.  This doesn't mean that they can't still try to collect from you, just that they cannot go to court and obtain a judgment.

The original creditor at this point has most likely sold the debt to the debt collection agency.  These agencies use many tactics of intimidation, including threatening you with court and trying to tack on ridiculous charges/penalties to the amount that they say that you owe.  At this point, you can either try to work out a payment arrangement just to get them off your back or just ignore them.  Note:  They would have purchased this debt for pennies on the dollar.  If you are inclined to work out a settlement, I would offer no more than 20-25% of the original amount owed.

If for some reason, they are able to sue (if the statute of limitations has been re-started as explained above), then write back.  There are several strategies that you can pursue in court that might get the case dismissed.

Best of luck.