Oh wow. I was going through the Anvil Runes for the Dwarfs, and I noticed something. Dwarfs have three standard Anvil Runes in 8th Edition's base army book, and a further three Ancestor Runes in the Storm of Magic Supplement, which are buffed up and more powerful, and I've come across something very interesting that I found relevant.
The standard Anvil Runes are:
Rune of Hearth and Home: Targets every single Dwarf in 24" (and only Dwarfs). Grants Immune to Psychology for a turn.
Rune of Oath and Steel: Targets a single Dwarf unit, giving +1 to their armor saves for a turn.
Rune of Wrath and Ruin: Cracks the ground open to vent magma at enemies, dealing 2d6 Strength 4 hits to a unit distributed as shooting.
Fairly tame stuff, but I think you can cast all these runes as bound spells once per turn each, so it makes sense that it's limited. Now, here's the big stuff. The Ancestor Runes:
Ancestor Rune of Grungni: Targets a number of friendly units equal to the number of successful strikings (notice that this isn't restricted to Dwarfs like the regular Anvil Runes). All allies affected by this have the Fight in Extra Ranks special rule and reroll all failed to hit rolls for both shooting and close combat for a turn.
Ancestor Rune of Grimnir: Same as Rune of Wrath and Ruin except it has a wider area of effect.
Ancestor Rune of Valaya: Targets a number of friendly units equal to the number of successful strikings, not Dwarf restricted. Heals targeted units in the exact same way as Regrowth and grants all affected units Unbreakable for a full turn.
Why is all this relevant? Well, we've seen Anvil Runes in the quest before, but here's why I think it's important:
What Kragg did here was not Oath and Steel or Hearth and Home. Aside from the fact that those Anvil Runes only affect Dwarven units, the effect presented here does not fit what they do. He used the Ancestor Rune of Grungni on them, which grants fight in extra rank and reroll hits, represented as "Dwarfish enmity of greenskins" strengthening their arms and flooding through their minds. That might also explain this:
Kragg didn't just use a regular Anvil Rune, he got so caught up he used an Ancestor Rune, a greatly treasured and powerful artifact of the Golden Age to empower the Undumgi. I can understand Belegar wanting to cover that up to prevent outrage from the wider Dwarf community.
This has been pointed out a long time ago, but it's also the reason why Mathilde noted the Undumgi had such an affinity for Dwarfs and picked up Khazalid nearly as fast as she did. Their souls were literally transmuted with the spirit of Grungni.