However, the key thing here is 'when the stars align'. The Heart talks about how when the stars align, it will emerge from the shell of the earth, annihilating it. The Heart's statements are dodgy, which would cast doubt on tier 5 in the first place.If they were at all comparable, the planet bust shouldn't be inevitable. The current reason for this scaling is 'it's not known how close the heart is to that goal', but that doesn't really matter. You wouldn't scale to something you have no hope of fighting off. If we take the heart's statements as fact, we'd also have to take into the account that this is portrayed as inevitable, which means the party members don't exactly have a chance of stopping it. That really shouldn't be scaling to the party. Let's for a moment assume that the Heart of Darkness is correct in that it will eventually blow up the world. However, the latter part is problematic, and really should not be scaling out to anyone.

Low 7-C scales off a feat a boss did, with 5-A scaling off the Heart of Darkness eventually blowing up the planet. So, currently late game Darkest Dungeon has 'At least Low 7-C, possibly 5-A' as their tiering.

Darkest dungeon endless virtue