[Coldsaturn]

Well, "Lemuria is plunged into endless darkness." is the immediate consequence, and "many years later than the Sea God has perished, when the kingdom has sunk to the depths of the Ocean" does imply that something has gone irreparably wrong. This story's pivotal point is the Ceremony, so if the text is mentioning it we can presume it's related to that event rather than something else in the far future. The flame that Raf carries out might have been his own evol, and not the actual Lemurian sun, because everything just crumbles after that.
Also, we know from Nightly Strolls that Raf's whiny monologue once MC finally comes to him to the hospital probably has more truth to it than simple bratty lines "It's been 800 years" could very well be literal for him, since it's close to the span of time between one Ceremony and the next. We also know that he met MC in the present when she was a child and he was also very young, and at that point Lemuria was an underwater ruin. After the Chronorift induces a tsunami that brings Lemuria to a depth that humans can explore, the civilization has long since abandoned its home.

It could be that the Ceremony didn't actually work because Raf was supposed to take MC's heart, not give away his. And the sea god needs the follower's heart to power lemuria's sun and properly use his power.