The great thing about that is, there are 5 enchanted skulls that the characters must collect. So, whether they like it or not, the characters are going to go into dangerous situations, trading present comfort for future gain.
Sometimes one of my players will say “but I don’t want to go to x location,” and I shrug, because I know they will want to go. Why? Because in their list of options, going in that direction will be the least unpalatable.
“I dislike death, however, there are some things I dislike more than death. Therefore, there are times when I will not avoid danger.” – Mencius
What do the characters desire more than survival? At the beginning, or in a grubby dungeon crawl, that answer is loot–and more better loot. But when you get to the point where the characters are more realized, where they are invested in their setting, then offering them a way to make life better for those who will otherwise suffer needlessly… That is when the possibility of what is “loot” expands in helpful, story-building ways.
A cup with jewels is expensive and nice. A cup that can heal the suffering king and the whole kingdom? That’s better. But to stop at this level of insight is to risk missing the biggest point.
Because the greatest treasure is when characters belong in a world strongly enough that they will sacrifice for it with no thought of reward, because the world is valuable to them. They have a chance to protect what they have come to cherish.
In short, the world is the loot. The McGuffin is a way to protect that loot.
If this is a useful insight, then the question becomes, how can I make their game world, its diversity and beauty and culture and history, the thing they most want out of the game world? Then you’ve got something special.