For any dismemberment you need your whole character to be set up for it with a split mesh.
If using mr mannequin you would take it to blender, edit the mesh and separate by loose parts.
Then you could import the parts one at a time associated with the same skeleton.
Then in the character BP you would recombine the parts.
And you need to sync up the animation between all the meshes.
Then, obviously, once you dismember you can disable the sync of the animation and maybe replace the mesh with a different bloody version after some kind of particle effect to hide the switch. Or spawn a decal. Depends on what you are going for.
Many tutorials out there on how to stitch parts together and sync them. Look those up and you should be able to get it rolling 
Ps: ita a bit performance intensive, but if fortnite can run on mobile with a fully blinged character this shouldn’t really affect performance at all.