When breeding a creature that triggers a ritual, you can encounter a odd lock situation:
Have one creature at the stable
Have one creature in your party
Breed the creature in your party with the creature in the stable
If you have less than 1500 Brimstone or Crystal, you are unable to summon another creature from the altar
With no creatures in your party, you are unable to venture to a realm to acquire more resources.
With no creatures in your party, you are unable to use consumables to complete the egg ritual.
And… as soon as you breed, the game autosaves.