- Master OR Misfortune
It may seem similar to Disaster, but Disaster implies a change, while Misfortune doesn't.
A Disaster is for children to lose their parents. A Misfortune is the life of orphaned children.
This is not too well a story for RPGs. The Elements are not usually the taste of most players and to be witnesses often will change the story into a Supplication or Rescue. If they don't try to change it, then they don't care and there's no story.
The Unfortunate must be (a lot) less powerful than the Master or not powerful enough to overcome the Misfortune.
PCs as Unfortunate
This is, at most, a starting point for the PCs. No player wants to be in this situation for long. If they ended under somebody, for instance, after a Disaster, or if they started in this situation, for instance, by starting as slaves, they will try to escape it.
Not every Misfortune needs a master. To live in a poor land, difficult to cultivate, is a Misfortune without Master.
If there is a Master, it doesn't need to be united. The slaves' life in ancient Rome was a Misfortune, but there were not a society of slavers but several unrelated, un-associated masters.
If there is a Master, often will be satisfactory to take him out, but you can do more complicated things. For a Misfortune story, a Master does not need to treat its slaves badly if the lack of freedom is enough by itself.
The Master needs not even be conscious of the suffering of the Unfortunate.
PCs as Master
This is not usually a place most players are going to feel fine. They don't want to feel like the bad guys, so you need to make it feel normal, like everybody does the same, like nobody cares. The problem is that, if the PCs don't care, then there's no story.
Most Misfortunes are based on feelings. Lack of freedom, not to be considered a human being.
A plague can be a Misfortune without a Master, but this kind of stories are going to make your players suffer so, in any case, it will be a starting point.
If PCs are the Master and they care about the Unfortunates, they can free them (Deliverance) or look for a way to do it. It can be more complicated: in Dark Sun, where slavery is common, PCs with slaves may want to free them, but then the now ex-slaves could be captured and enslaved by others, so maybe the PCs need to understand how can help their ex-slaves to remain free (Deliverance), ask others to advocate for the end of slavery (Supplication), conspire against the slavers who would stop the end of slavery (Conspiracy)
When the PCs free their slaves, these can think that freedom is nice, but without good is hard to enjoy it, and despoils the PCs lands or goods (Fatal Imprudence while thinking about giving them liberty, Disaster or Vengeance after then)
In a lot of cases, the suffering of the Unfortunate brings a benefit to the master, so the PCs are sacrificing whatever they get by keeping the status quo (Sacrifice)
Whatever the Misfortune, if the PCs are the Unfortunate they'll want the Misfortune to stop (Ambition)