For cultural victory how about being able to sac your heroes for 1000
per level (i.e.10000 culture for 10th level)? Or 500
per level and a building that boosts your culture and your economy. That way their heroes would be useful beyond warmongering and it would enable interesting strategic choices (do I sacrifice him now? Will the remaining ones be enough to stand in war?) Perhaps the spell should need a certain time to be cast (about 20 turns) and should trigger a popup to give the opponents time to declare war on them.
Something like "Great legends about (hero name) are told in the Grigori realm and now spread into our lands fascinating our people - Some fear the people could lose their connection to our land."
If you want to extend it you could even make an event:
--> option 1: We can't allow the influence of those heretics --> DoW 10 turns of hurry anger option 2: "We won't do anything about it. These are just tales, nothing more." --> +10 Grigori culture in all your cities 10% chance for each city of removing all religion from it. option 3: "This hero should is an example of virtue. He will inspire our people and will guide us in distress. Construct a Monument of him in every city." --> Receive a Monument with +5 culture (like with the Grigori option in the Monument event) in every city. 50% chance for each city of removing all religions from it. +200 Grigori culture in all cities +4 relation to the Grigori leader


Something like "Great legends about (hero name) are told in the Grigori realm and now spread into our lands fascinating our people - Some fear the people could lose their connection to our land."
If you want to extend it you could even make an event:
--> option 1: We can't allow the influence of those heretics --> DoW 10 turns of hurry anger option 2: "We won't do anything about it. These are just tales, nothing more." --> +10 Grigori culture in all your cities 10% chance for each city of removing all religion from it. option 3: "This hero should is an example of virtue. He will inspire our people and will guide us in distress. Construct a Monument of him in every city." --> Receive a Monument with +5 culture (like with the Grigori option in the Monument event) in every city. 50% chance for each city of removing all religions from it. +200 Grigori culture in all cities +4 relation to the Grigori leader