On this topic, how does the Hammer of Sigmar prayer work against an opponent with the Rogue Helm?
No re-rolls of re-rolls, so the helm just auto-deletes that spell for attacks made against it?
Stick with the first roll, no re-rolls at all. From the FAQ.
Q: If a model is granted a re-roll To Hit from a special rule such
as Hatred but the model they are attacking has a special rule,
magic item etc. that forces successful To Hit rolls to be re-rolled,
how is this dealt with? (p7)
A: They cancel each other out and no re-rolls are made for as
long as both special rules are in effect.
I had this happen with with the Goblin spell Sneaky Stabbin' (re-roll failed wounds) vs a KDaii Destroyer (re-roll successful wounds). Just keep the first roll.