I haven’t tried it myself, but I imagine you could place dimming planes (flat plane with a dark translucent material applied), then use a simple vision detection algorithm (i.e. From this actors perspective, it can see ‘this’ distance away/this actor can see ‘these’ sections of the map), and simply toggle a boolean variable on each plane whether or not to render it.
Obviously not the best solution as it could take up a decent amount of resources, but if you optimise plane shapes and any LOD settings you could really make it work.
If you do give this a go, let me know how it goes for you 
EDIT: The method is obviously dependent on the viewing angle. Top-down style like league and dota would work, but you’d need to take into account the angle at which you’re seeing ‘under the plane’, like when you stand next to a table and can see a small portion of the floor underneath. Just food for thought.