New way to give bonus for demonic units

[Horyu]

I would suggest to give bonus for demonic unit using event offer for low tier units to sacrifice for a high tier unit to upgrade strenght or speed or UI or leadership bonus in a particular friendly city in a certain timeframe

eg.
A turn event may offer
1) 2 units of sea serpent sacrifice for 1 unit of medusa to gain +10 battle on swamp and swamp movement bonus
2) 4 units of orc sacrifice for 1 unit of demon to gain +3 leadership in a friendly city within 3 turns timeframe
3) 3 units of eagle sacrifice for 1 unit of devil to gain flyer in a friendly city within 4 turns timeframe