NPC: Prophetess Cavrylin
ID: 20683

There should be a periodic scripted event with her and infernal involved.

She should say "Another one! Bring me another one!", then an infernal should walk to to her and stand in the middle of the magic circle. Then the infernal should get launched.

You can also check the wowhead abilities to see transform for infernal(where they turn into a ball before launch), and launch ability on the prophetess.

https://youtu.be/D4l1mlTn-fY?t=37
https://youtu.be/sPQFt0AjHFs?t=55
http://www.wowhead.com/npc=21316/deathforged-infernal#abilities
http://www.wowhead.com/npc=21316/deathforged-infernal#abilities