this is like the praetors "Soul-forged Guardians" ability...
At the start of the first battle round, before determining who has the first turn, you can pick 1 friendly STORMCAST ETERNALS HERO on the battlefield to which this unit will be bound. Roll a dice before you allocate a wound or mortalwound to that HERO while it is within 3" of this unit.On a 1-2, that wound or mortal wound is allocated to that HERO as normal. On a 3-4, that wound or mortal wound is allocated to this unit instead of that HERO. On a 5-6, that wound or mortal wound is negated.
and in the latest faq it says that its considered a ward save.
Q: Some abilities allow you to roll a dice to attempt to negate a wound or mortal wound, but that roll may have a different effect depending on the score of the dice (as per, for example, the ‘Soul-forged Guardians’ ability on the Praetors warscroll). Are these abilities wards, and is the roll a ward roll?
A: Yes.