[Finalgenesis]

National spell: Manifest Great old one

[battlefield version]
Chance to manifest a great old one, failure can result in various effect like calling multiple horrors, random horror marking/insanity...etc. If successful, a random great old one is manifested for the duration of the battle, they have various powers and may not always help you...
[Ritual version]
Same as above, can only be cast at capital, may cause events that kill off your population / insanity ...etc. Success in manifesting a great old one generates random effects associated with them (Not all are beneficial).

Example: Ithaqua when manifested in battle would grant cold immunity to your troops for the battle, he also joins the battle with a handful of Shantaks. The ritual versions gives you various random possibility such as: a handful of shantak, a wendigo or two, global +1 cold scale shift, cold event at your capital killing off some pop, the great old one may even join you temporarily (at great cost like eating your pop or making your people insane) ...etc

Yog-Sothoth manifested in battle might cast vortex of returning or cast twist fate on all your troops. In ritual if you manage to summon it might come with the astral pathway power (like gate-stone) and/or eyes of god effect, though causing disaster events , random dominion loses and insanity on your commanders globally until it leaves. Other ritual effect may get you an avatar hero (say 'Umr at-Tawil) who is much less powerful but permanent and without the downside, maybe a "boots of plane" power like the visitor.

That's a lot of graphics and content to create for 2 spells of course but practicality aside, I can dream about it!