Slave rebellion in the Danish West Indies is an event for 
Denmark. It requires that Denmark owns St. Thomas in the west indies and that slavery is allowed. Both are true at the start of game. Then it can happen anytime between 1848 and 1852 with a mean time to happen of 8 months. The MTTH decreases by 25% after 1849, by 5% by 1850 and yet another 5% after 1851.
The player then has two choices:
1) Free the slaves which changes slavery to not allowed and costs 500£.
2) Crush the rebellion which gives all 
slaves in Denmark 10 militancy and 5 consciousness. On top of that it gives all liberal POPs 2 consciousness.
Note that option two gives militancy to all slaves in Denmark, not only those in West Indies. So slaves in Danish Ghana will also become rebellious, as well as other provinces, that Denmark has conquered and extended slavery to.
It cannot happen to 
Scandinavia even if it is formed by Denmark, and they own the now Scandinavian West Indies.