Declare a Mortal Enemy/Holy War

Similar to how an Alliance is one step above Peace and offers more benefits, a Holy War or declaring a Mortal Enemy would be one step above War, like a high-risk, high-reward war where you get more benefits if you do well but suffer harsher consequences if you don't. If the various political states were on a scale, it would look like Alliance - Peace - Cold War - War - Holy War

Possible Constraints:

• Can only be declared against one empire or alliance in any game
• Must be within certain score or military power threshold (i.e. 10% of the player's score to prevent the top player from abusing the bottom player)
• Costs a very large amount of influence
• Two empires or alliances who are in a holy war or mortal enemies of each other cannot be in a holy war or mortal enemies of anyone else (to prevent dogpiling)
• Since Horatio and Vodyani are heavily religious, they might get extra empire attributes that reduce the influence cost of starting a holy war
• An empire that declares a holy war cannot declare normal war with anyone else, but others can declare war on them

Possible Risks & Rewards:

• +5 approval on empire for 10 turns per battle won/-5 per battle lost against mortal enemy
• Income from blocking enemy trade routes
• If empire offers truce, they have to pay a lot more reparations per turn
• No approval penalty for refusing truce
• Huge approval bonus for the victor
• Huge religious support for holy war, especially if you win
• All parties besides religious and military disapprove of Holy War
• Decreases the disapproval threshold before rebellion occurs (easier to slip into anarchy)
• Zealous Citizens: all FIDSI increases the more you succeed in the Holy War, but decreases the more you fail.
• Heroes and Ships gain double XP in Holy War battles

What do you guys think? I like the high-risk high-reward potential this offers, allowing two competing empires to try to get an edge over the other, risking major failure for major victory. This also, I think, might help in certain games where two empires are competing over one constellation while another empire might have their constellation all to themselves. Whichever empire emerges victorious over the other in the same constellation won't be so far behind the empire that had no trouble expanding throughout their whole constellation with the bonuses of winning the Holy War.