First off, head to the south well and kill all the sirens (beware - they aggro and paralyze if you're not invisible) until you are left only with Thyria, the siren lieutenant. The person who needs the mark should attack her, she will speak, say yes to her and let her talk. Once done head to the north well, go invis and head up to the queen at the top. The queen is in a NPK (the only PK room in this whole run). She will speak, after she's done (leave quickly or she will attack you) head back to the south well.
Be sure to be invis then head back up to Thyria, have your high level friend go into the room before Thyria first and tank the siren bodyguard (aggro, no-flee) that has appeared there. Once it's dead, have your high level friend attack Thyria and weaken her to make her easier to kill as she hits hard and will para. She is also pretty easy to kill with a decent form (3-4) of 120s. Make sure everyone flees before Thyria dies, because the person who talked to her to begin with must get the killing blow, or you will have to start over. After she dies you will be awarded the mark and teleported to Towne of Rune recall.
If you have already completed the mark, killing Thyria will teleport you to the Siren Queen. Also, if you have the mark and enter while others are in the middle of the run, it tends to reset the run and you will have to kill Thyria and start over.
For having the humility to change course, the Powers have granted thee a Mark. You have been granted The Mark of Reversal! You gain 36 practices!