Per the comprehensive rules, it should be reduced to 3.
In the app? I haven't encountered it so I can't confirm. If you saw it not get reduced, that would be a bug.
I could be embarrassingly, horribly n stupidly wrong here, but as I see it...
Rend: Damage that is applied with "Rend" bypasses any damage reducing effects.
BWK: Damage from highlighted Units cannot exceed 3 (after damage reductions).
The Barrows Wight King effect does not say "after OTHER damage reductions", nor does it use the word "reduce" to describe its action, so it would appear to NOT be a "damage reducing effect" per se and therefore Rend does not apply and the maximum damage is 3.
BUT neither 'increase' nor 'decrease' are Keywords as defined by the Comprehensive Rules, nor are they listed as game terms either, so I can see how people could argue the point either way.
Well right now BWK seems to not be working in app even with non-Rend damage. I've logged a bug about it.
@LankySi it definitely makes any damage higher than 3, 3. It is NOT a damage reduction effect.
The badge isn't up but I'm a Heraldor.
Yes @LankySi. Zombie Dragon
Corner 1 X
Corner 2 2 rend damage
Corner 3 4 rend damage
Against a BWK would do 2 then 3 damage assuming no other factors.