If your party has either an Adept or an Exorcist (or both), they can learn Dark Magic up-to Expert, Level 4. You must find the person who 'sells' the Dark Magic Skill, and also find the person who sells membership to the Dark Magic Guild. Buy the skill and the membership, invest four skill points in that skill, find the Dark Magic Expert Promoter, get promoted. Go to both Dark Magic Guilds and buy spells. Then go see Master Su Lang Manchu in the Temple of Dark in Tuonela to receive the 'free' Dark Magic Master promotion to Level 7. However, you **must** first be an expert in Dark Magic or he won't promote your Exorcist and/or Adept.
Also, if your party has an Elder, he will receive Dark Magic Expert Level 4 from Du Lang Manchu.
I personally would not go into the Fortress of the Ancients without this skill.