Ouch.
Hmmm, perhaps I should keep my mouth shut about breeding program earlier.
Nifelheim's counterattack was very effective. He teleported more than half dozen tartarians and his pretender, all equiped with powerful battle gear, on top of many uncastled provinces that I have raided last turn.
In all but one province he won battles easily, since my tartarians were scripted to cast returning or retreat.
But I have prepared nice ambush for him in one province, teleporting two of my best tartarians there to add to the one HOF tartarian who already was there. Together my three tartarians were scripted to ambush and destroy whatever counterattack team Niefelheim would send against them.
Well, the ambush had worked, all right. They caugth up the great Seraph of Fire, Uril. (Seraph! In service of Niefelheim.

Got it?)
7F, 8S, 5A, equiped with 2 unique artifacts - Mage Bane and Solar Brilliance.
The bloody thing had destroyed my kickass tartarians, all three of them, without breaking a sweat.
All I can say - I am glad I have decided to go after giants now and not later.
Well played Niefelheim. I will have to be more carefull in the future.