The Cult Magus is a Unique Hero available to each monogod race. They are only available in campaigns, can’t be put into armies or garrisons, and their only real purpose is to establish a Chaos Cult in an enemy settlement. Once a Cult Magus establishes one cult, they disappear or “die” permanently; they are single use Unique Heroes.  This means that you don’t have to use actual heroes to set up these cults and don’t have to waste any of the actual heroes’ potential. 

There should be Cult Magus-equivalent Unique Heroes for Skaven and Vampire Coast who are only available in campaign, can’t be put into an army or garrison, and who’s only real purpose is to set up an Under-City or Pirate Cove, respectively. Each one of these should also be single-use and disappear or “die” permanently after setting up one Under-City or Pirate Cove. The Skaven can have an Under-City Engineer to establish Under-Cities and the Vampire Coast can have a Vampire Smuggler to establish Pirate Coves. 

Additionally, either Vampire Smugglers should also interact with the Treasure Maps and dig for treasure or there can be an additional Unique Hero for the Vampire Coast, a Vampire Treasure Hunter, making a low-cost option to send on treasure hunts with the Treasure Maps. Vampire Smugglers or Vampire Treasure Hunters should remain and not disappear after digging for treasure and completing treasure hunt with Treasure Map. A single one of either should be able to be used to hunt for treasure with Treasure Maps for your whole campaign.

Skaven do already have this, the DOOOOM! Engineer.

Works pretty much identically to how the Cult Magus does, automatically creates an undercity with a unique building and kills the hero. The Cult Magus takes ten turns to summon (from an already existing cult) while the DOOOOM! Engineer is summoned by a rite with a cooldown of 30 turns (that could maybe be reduced since it seems a bit high for what it is).

Vampire Coast could probably do with something similar, but frankly that's the least of what VC need to make them better.