This isn’t a joke from a cracker 
…can they be recruited, or best avoided?
A Balrog dwells in the mountains on the south side of the Gap Of Rohan. A DS army can recruit him if you enter the same hex. It kinda happens automatically. There is no prompt. FP and Neutral armies will lose some troops for the same encounter. This Balrog does not offer individual encounters.
Durin’s Bane is found usually around Khazad-dum. He is an individual encounter. Enjoy the combat with this one. He’s a bruiser. He does engage with armies.
Great, thank you both!
Learning about the game all the time! 
Simon