From Destinypedia, the Destiny wiki
Nightmare Bellkeepers are the Nightmare Realm counterpart of Honored Bellkeepers. They manifest soon after the players enter the Nightmare Realm and don't move far from their starting positions. Like their waking counterparts, their purpose is to keep the Bells from being rung. Generally, killing a pair of Nightmare Bellkeepers will unlock a bell for ringing.
In the first two encounters, they appear around the side where a bell wasn't rung. In the final encounter, two pairs appear around the bells that weren't rung, in which case they all must be killed to ring the only bell present. While the Nightmare of Caiatl is damageable, three pairs will appear around each bell, each of them blocking the bell closest to them.