Well, let's compare them side by side.
A holy avenger is legendary, a sword, and requires attunement by a paladin. It grants a constant +3 to both attack and damage rolls. It deals an extra 2d10 radiant damage to fiends and undead. It effectively grants the Magic Resistance ability to allies within the paladin's aura, and expands at (roughly) the same time the paladin's does.
Your brutal destroyer is legendary, (any weapon?), and requires attunement by a barbarian. It grants a +3 to both attack and damage rolls. It deals an extra 2d10 necrotic damage to celestials and fiends. It has a chance, based on the character's stats, to confer disadvantage against magic to all enemies within an aura, which expands at the same time as the Holy Avenger's.
So, sentence by sentence, our comparison:
The brutal destroyer is slightly more variable than the paladin's longsword, which isn't necessarily bad, but I would recommend for flavor that it be limited to the barbarian's iconic weapon, the greataxe. I'd also strongly recommend at the very least limiting the brutal destroyer to melee weapons if nothing else, as having a ranged version of the holy avenger is definitely unbalanced.
The damage output via magic is similar. Just as the two types targeted by a holy avenger typically have no resistances against radiant damage, the two types targeted by a brutal destroyer have no resistances to necrotic damage. Most parties are more likely to run into undead than celestials, but the two types targeted by the brutal destroyer make sense if you consider the barbarian wielding it to be a champion of nature.
There are multiple problems with the aura. Rather than providing a passive benefit to allies, the brutal destroyer lowers resistance for enemies. The DC for an ability granted by an item is normally fixed. While the paladin's aura meshes with and enhances his Aura of Protection class feature (which is why a paladin must be the one wielding it), the aura produced by the brutal destroyer has no analog in the barbarian class. Perhaps more importantly, such a feature doesn't actually help the barbarian themselves at all; a barbarian in a fight is almost certainly in a rage, and thus can't cast the spells they are empowering.
Suggested Fix: An aura is fine, but it should be something that meshes with and enhances one of the barbarian's standard features. Options include allowing allies within range to gain some of (not all) of the benefits of your Rage- and you should be specific about which benefits are gained- granting nearby allies the benefits of Feral Instinct, and, in what is probably the best fit for your weapon, granting nearby allies a benefit similar to Brutal Critical, wherein they deal extra damage of some sort on a critical melee attack. I personally would go for you and each ally within 10 feet dealing an extra 1d10 necrotic damage with a successful critical hit with a melee attack, with the range increasing to 30 feet at 17th level. Call it an Aura of Destruction, to contrast with the Aura of Protection.