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Because you need to look at the theme of the "Invasion from the Underworld" campaign, not the map itself. Yes, this is the fourth mission of the campaign, not a standalone map. Master usually created such topics as he developed the campaign, and the need for them disappeared with the full release.
And yes, this campaign is one of the most difficult, if not the most difficult, in the community. Master's name has long become a household name due to the hardcore nature of his work.