We were doing the Enchanter’s Lair in Cataclysm with the event. We clear the ritual and pick up the skull. We managed to reach Nurgloth and did some damage during his first fase. At some point he just stopped taking damage. Nothing would do anything to him. After a long while trying to do a single dent in his health bar we decided to try to drop the skull of the event. Then instantly his health lowered and entered the next fase normally.
On the first 2 pictures you can see in the lower right how long it went without him taking any damage. And on the last one, you can see how without the skull his health lowered.