Action activates when all players but 1 die?

Make an alive variable inside the players, and create a separate script to handle switching it off when their Humanoid.Died

Then create a table, like alive = {}

alive = {}
--later in the script
        for i,v in pairs(game.Players:GetPlayers()) do
            if v:FindFirstChild("Alive") then
                v.Alive.Value = true
                table.insert(alive, v.Name)
            end
        end
--this is the part to end it If only one player is alive
        for time = 1,120 do 
            wait(1) 
            if #alive <= 1 then
                break
            end 
            for i,v in pairs(game.Players:GetPlayers()) do 
                if not v.Alive.Value then 
                    table.remove(alive, i)
                end 
            end 
        end 

This is just an example of how I do it.

Yes, you would use a Died event, and the variable that holds the players should be a table. Whenever a player dies, you would remove them from the table, and after a player dies you should check the length of the table that holds the players and see if the length is equal to one, if so, do your action.