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7 weeks. If Guard appeared, it means Jezebel has already been defeated. He only appears after her death.
Were you with the amulet or without it?
(Yes, that battle in the Temple is not a bug; just kill all the Hydras, including the summoned ones, and it will end). The amulet is very important for raising vampires, and some resources and money are in the treasure room. + a key to Blindness.
You need to defeat the Green one earlier. Then you will get the Tear of Ash, take it from Duncan's castle, and place it in your castle. And yes, then you will be able to build the Temple of the Underworld.
180 vampires is not enough. And you also need to accumulate the rest of your army. Death Knights and Death Messengers are very desirable.
With Guard, you can defeat him. Darkness will help. Resistance and the Armor of the Forgotten Hero will help. + Seal of Darkness. Vampire Princes and Death Knights can also help if they reduce the demons to half health, and of course, Spiritual Link. I defeated him in about 5 attempts, in the field.
The tactic of distracting the demons with single Vampire Princes worked. + then don't forget to cast Blindness on the Imps.
And one more piece of advice: you can't win with just one hero here. You physically won't have enough time. Level up both Cole and Stefan.
Resurrect Stefan as early as possible - this will give you more time to maneuver.