It would be wise to consult an attorney in your area. The attorney will know which documents to file on your behalf; it will likely be what is called a Special Appearance. The point of the SA is to let the court know that it does not have jurisdiction over you or that you are not/should not be a party to the case.
I ran into this issue once before, and a call to the attorney who filed the action cleared it right up. All we had to do was provide documentation that the lien had been satisfied and that we were no longer asserting any claim.
This sort of thing happens from time to time on Quiet Title Actions, especially from Sheriff Sales and Foreclosures, and shouldn't worry you too much.
I would definitely advise dealing with it, but don't be too emotionally traumatized or bother, since it's usually just a formality observed when someone wants to clear title after obtaining the property in an unusual way.
If the attorney won't deal with you, you can hire an attorney or even represent yourself in the action. Once you explain the facts and show documentation of the assignment, you should be released from the case.
Attorney answers to questions are for general purposes only and do not establish an attorney-client relationship. This answer is meant for informational purposes only and should not be construed as legal advice within the bounds of a professional relationship. It is always best to seek counsel with a competent attorney experienced in your area of issue and fully informed about the facts of your case.