I was robbing houses, pickpocketing and so on. Then I got into Windhelm's Alchemy & Enchanting house with around 4 NPCs in it,
One of them attacked me and I've noticed that NPC's power attack was a bit like bigger, wider and longer Fiery Breath (DK) and he was firing it from his hands.
Now I'm aware that there are many other attacks from humanoid enemies we might've seen but in truth I've never seen this spell in the game, not even from enemy or ally.
First off I thought that it might have something to do with Spellcrafting, implementing new stuff to the game earlier, just like it is with Motif: Glass that was added a couple of patches ago without any news, it just got into the game's files quietly and just sat there, tho AFAIK there is no craftable glass armor, just to avoid speculation.

Have you ever encountered any similiar ability, you've never seen? Mind me, only friendly NPC's using it. Especially when they're attacking you due to breaking law.
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