I was waiting for a Kara raid in Duskwood, and I decided to have fun with these guys.  I put Curse of Doom on them and feared 'em.

Curse of Doom hit for 5094 damage.  It took away 1% of his HP.
I put CoD on again, but it was immune.  I Death Coiled.  Immune.
He attacked.  This is what my Combat Log said:
"Black Rat hits you for 9999 (glancing).
Hathuun hits Black Rat for 1.
Black Rat hits Hathuun for 15,982.
Hathuun dies."
Now I was paniced.  I popped my Defender of the Timbermaw.  Again:
"Timbermaw Ancestor's Healing Wave critically heals you for 700.
Black Rat hits Defender of the Timbermaw for 19,005
Timbermaw Ancestor dies.
Black Rat hits you for 25,000.
Your equipped items suffer a 10% durability loss.
You die."

So watch out.  You might need some sort of 40 man raid with really good healers to take these big meanies down...  :P