Well, apart from the fact that the art looks pretty good from the preview shots, I don't have a whole lot to say good about him, other than that he inspires a very interesting theme deck.
No denying he's huge. With no penalty for his loss, I'd have to say this guy is about the ultimate security net for going through the underdeeps. He should basically never need to tap, so he should be able to take strikes all day long. The 4MP ally is a big bonus, but alas in MELE there are so many non-unique allies that it gets hard to double people there; for that reason, there are obviously a lot less painful sources of MPs. You can't even use Reluctant Final Parting to try to clean out your opponent's allies for two reasons: a) the wolf and warg allies are not very suceptable to this card, and b) it will always discard the Balrog itself.
However, there is a key problem: it requires a huge character to keep this guy in check. There are no 10 mind minion characters, so one is forced to look at the 9-mind characters: The Mouth, the 3 Lieutenants, and Golodhros. Even these guys, however, stand a 1:36 chance of getting eaten every turn. While the odds aren't great, they will catch up with you before too long. While I have found plenty of MELE cards that _decrease_ a character's mind, I haven't yet found any that _increase_ it. And unless you have an underdeeps-heavy deck, you hardly want to waste a big character to just watch over the Balrog, especially since he _will_ get killed eventually.
So, let's say you send The Mouth to babysit this guy, so you can just leave him watching the Balrog somewhere safe (say, the Under-courts) while he recycles cards. What else is there to do in the Underdeeps? Well, the most interesting sites are the Gem-Deeps and Pukel-Deeps, both of which are Ruins and Lairs with items of all types playable. The Under-grottos still have their +1 to ring tests their, which makes them interesting. The Under-Leas and Under-Vaults both have major items playable, but they certainly are painful ways to get major items. The rest of the underdeeps are pretty useless from an MP-gathering perspective. And let's not forget about the ream of hazards that can kill you down there. I would assume that The Balrog is a manifestation of The Balrog of Moria, although the spoiler in Inquest doesn't explicitly state that, so at least you don't have to worry about Durin's Bane. Finally, there is the Ancient Black Axe and Iron Shield of Old, for a total of 6MPs playable at Under-Deep shadow holds. Not too shabby, and these two items provide some decent table advantage. While the Black Axe is no Aiglos or anything, it is certainly powerful. The Iron Shield of Old is less compelling
Let's also not forget that before you can convince the guy to help you out, you first have to go to the Under-gates and face him. This is one of the most hazardous sites in the game, quite possibly the most hazardous, for both Minions and Heros alike. At least you could take an extra Major item for your troubles.
I would say that if you are a minion company, and you are planning to spend all of your time underground anyway, the Balrog could be a very useful addition, and definately inspires a theme deck. Assuming he doesn't kill your most powerful character and leave, costing you 7 MPs, he could be very valuable for protecting your company while down below (assuming he actually shows up in time, as with all unique cards). He could also be a very good sideboard card if you opponent is spending a lot of time in the under-deeps ... I wouldn't want to have to face The Balrog in company-to- company combat (and the very interesting MEAS card World Gnawed by the Nameless can really allow Minion companies to zip around down there, nailing unsuspecting heros). He would be an ideal ally for longer games, when the Under-deeps become more practical, if it weren't for the fact that you have to leave such a large character watching him when your companies want to resurface (and in a longer game, it is unlikely you will want to spend the _whole_ time underground).
The reality of the situation, though, is that there are about a bazillion easier ways to get 4MP than to deal with a Balrog; while the table advantage the Balrog gives you is significant, it comes at a very large price.
Original card review taken from : http://fan.theonering.net/morgulrats/
With the authorization of the webmaster.
The reviewing team consisted of Gwaihir (Chris Farrell), Gimli (Nathan Bruinooge), Ohtar (Charles E. Bouldin, Esq.), Radagast (James Kight), Joshua B. Grace (Beorn), Martijn Steultjens (Fram Frumgarson), Jason Klank (Saruman) and Jeffery Dobberpuhl (Wormtongue)