I didn't say you needed a corresponding spell to summon it. What I set for Rituals was that the player had a choice to give their Solar Legacy a ritual spell given it had the set conditions for a free spell. They don't have to take the extra spell, especially if it could already be summoned with something else. Or they could use a custom slot and make a better ritual spell with a graveyard effect. Or something weird, like the Amorphage field spell that's also a ritual spell.