Someone was able to bring the quest pet “Enthralled Val’kyr” into a level 10-19 battleground. the pet was hitting people for about 10k and had around 11k Hp. It was a Death Knight so I am not sure if it is a part of a Death Knight quest.
I’m UH DK and I have one…it’s the last talent at level 50…" Summon Garygol" , you can choose that or the other two. I use mine on big bosses…and I have used it in PvP BGs, but I’m Alliance…lol 
…but it’s also last resort… especially in a “Rare”…there is no quest for it…hope it helps…
appreciate the response, but it’s a summonable quest NPC, not the unholy death knight talent. That’s the one that was summoned https://www.wowhead.com/npc=45080/enthralled-valkyr#comments