Upon completion of quest the following occurs:
- Thank you again, <name>, for this delectable incense.
- Festival Scorchling devours the incense. It's ravenous!
- So good! So packed with energy!
- It starts growing in size.
- It has everything a growing scorchling needs!
- I can feel the power SURGING within me!
- Festival Scorchling bellows with laughter!
- Now! Finally! Our plans can take effect!
- KNEEL, LITTLE MORTAL! KNEEL BEFORE THE MIGHT OF THE HERALD OF RAGNAROS!
- At this point for a split second he becomes Wisp of Ragnaros.
- YOU WILL ALL PERISH IN FLAMES!
- He quickly loses strength becoming once again a tiny scorchling.
- Festival Scorchling blinks...
- Ah. I was merely jesting...
Therefore it's almost safe to assume those are actually servants or spies of Ragnaros. The whole sequence is very similar to what happens when a warlock completes the quest (Impsy enrages and begins to pyroblast his master Niby but before he can complete the cast his power abandons him).