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Basically, here's my idea: a Portal. But it only allows passage if you sacrifice a certain number of developed creatures (15 Overlords or Broodlings, 35 Nightmares or Warhorses, 60 Succubi or Temptresses, 150 Cerberuses or Hellhounds, 250 Elder Demons or Fire Demons, and 350 Imps or Parasites). So, I'm thinking it works like this: you enter the portal, and a message appears saying that a specific sacrifice is required to proceed. If you have the required units, the message asks, "Do you agree to sacrifice these units?" and if you agree, you can pass through. Is this possible? I've named the portal Shio.
P.S. Please send the answer via private message.