I definitely like the idea - thank you for posting (though I would rather not have to go to a link read about it). It always bothered me that only the bad guys / NPCs had access to world shattering rituals (like bringing Tiamat to the Prime). So I am all in favor of the concept and I like your general approach with the focus/sacrifice/recipe/effect/greater effect. The spell slot spending is a good idea too. That being said I think you could push it a little farther. Here are my thoughts:
1) My limit it to CR 20, I think you could easily expand this CR 30 territory. D&D lore is riddled with the idea that mortals can summon Arch-fiends or even gods. Now, controlling them is another matter entirely. But remember several demon princes have been imprisoned by mortal wizards. Why can't the PCs do this as well?
2) What no human sacrifices? Seriously rather than adding more this or that to get greater effect - up the ante. Make the sacrifices more evil (if we are summon fiends - which your sample is). Maybe human sacrifice is reserved for arch-fiends, but I bet a pit fiend would appreciate it. Which brings me too...
3) What if the caster provided better sacrifices for better chance of success - a bonus to the ability checks or a reduction in spell slots or something else entirely.
4) I find it odd that celestial and fiend blood are needed for the ritual. I could see one or the other in small amounts, but the lore of fiend ritual suggest animal and/or human (humanoid) blood is the way to go. Personally I see starting with animal blood and/or sacrifices and working up to humanoid blood and sacrifices for the greatest effects,
5) I like the idea of the recipe and the skill checks involved; however, this seems a little complex to summon a CR 10 fiend. I would suggest a simpler recipe. Maybe the recipe gets more extensive/complex as you reach for greater effects.
Good ideas and thank you for posting.
