Shadow and Flame
Not many people are aware of the existence of the Dwellers in the Dark. There's a good reason for that - most people don't survive the encounter. They are, much like the Balrog, a creature of ancient evil, one that has stalked across Middle-earth for countless millennia, slaying many Elves, Dwarves and Men along the way. Like their larger more-fiery cousin, these creatures fled to the depths of Moria after the War of Wrath, where they lay dormant for many thousands of years. The arrival of Dwarves in Moria, followed by the even noisier Goblins, awoke these brutal creatures from their slumber and they now stalk the paths and tunnels of the mountains in search of tasty prey (incidentally, Goblins are not that tasty, though they will suffice when there are no Dwarves left to eat).
So what can these evil creatures do? Well firstly, they are incredibly potent in combat. With a Fight value of 7 they are better at fighting than most enemy heroes. This means that in the event of a drawn combat they should win it (unless you've been charged by one of those pesky Elven characters such as Gil-galad). Of course, your opponent won't want to lose a round of combat against it because it will inevitably hurt, so they will more than likely use up valuable Might points to win the combat instead. The good thing is, these dark creatures have a high defence and plenty of Wounds to survive the encounter. They also have the intensely irritating (for your opponent) ability to regain lost Wounds when they cause damage, making them a very durable and extremely nasty addition to a Moria army.