If this is general, I would get an Acheron. It's good against the slime and gremlin family (especially gremlin).

There are 7 weapon lines that do shadow damage: Acheron, Gran Faust, Sentenza, Biohazard, Umbra Driver, Graviton Vortex, and Dark Retribution.

What is useful depends on whether you're thinking of FSC, the whole clockworks, or all that and PvP, etc. PvP is not my cup of tea, so Imma ignore that since I don't even know what is in vogue there.

If we're talking about FSC loadout, it might be a tough choice between Shivermist, Blitz, and Cold Iron Carver (or Leviathan) since they are all highly recommended by others for the boss fight. A Shadow-type weapon would mostly be dead weight in FSC, so it's not really wasteful to have another weapon for a type you already have covered. A lot of poeple are unimpressed with Alchemers, probably because they are late bloomers in terms of star level and require different techniques, but if you are also unimpressed with DA then upgrading your Alchemer to find out is kinda risky and the Needlers even more so. _IF_ you chose full Vog Cub over hybridising with Nameless because you want to specialise in swords, I guess the Calibur is the safest bet for Vanaduke's mask phases. I would also recommend considering using one piece of the Snarbolax set normally if you have not already, since Elemental/Shadow hybrid armor is popular for FSC, though strait-up Elemental/Normal is also popular and Fire resist is a lot more useful than Freeze there.

A Shadow-type weapon might be among the least useful things to add for FSC, but if OTOH we're _not_ talking in FSC-centric terms, a Shadow-type weapon is probably the most desirable thing to add. +1 to Soulstaker's suggestion here: If you find Glacius feels more right than Divine Avenger, then Acheron should be a safe investment because it's in line with the other Brandish upgrades. Since the fursuits incentivise sword use, and any Pulsar is a great "only gun you need" for a sword specialist, the Nightblade line would be my first recommendation. As for the other Shadow weapons... Graviton Vortex deals Shadow-type damage, but as Juances said, that's really just an afterthought. It's like the Vapourisers in that respect. If you don't like DA, you probably won't like GF unless you really want something to kill yourself with at the end of a run in order to share heat with friends who are always heating something.

That leaves Biohazard, Sentenza, and Dark Retribution as the most logical alternatives to Acheron. The choice between Acheron and the rest comes down to whether you would like to emphasise swords and leverage the Vog Cub bonus against Gremlins and Slimes too, or expand into more different capabilities and gameplay. I should point out my probably heavy bias in favour of the ToxiCat line, since IMO it feels like the perfect anti-Gremlin tool but I haven't seen many others reporting getting the same impression. My Nightblade is mostly collecting figurative dust at 3-star since it doesn't offer much vs. slimes either, since my Blazebrand is still powerful enough to make me think of hot knives and butter. I am probably in a tiny minority here, and your luxurious sword ASI probably renders this moot, but for what it's worth I also like the Roarmulus guns and Alchemers as backup melee weapons when placement and timing requirements get uncomfortably tight, so take a big grain of salt- like one of those water softener pellets -with the following...

With that disclaimer, unsurprisingly my second recommendation is Biohazard. If you like Polaris and know how to handle it in a party of other sword users without enraging them, you should be able to handle the Biohazard pretty well. Poison and insta-kill kabooms are very useful against healers, and having both options means you're less likely to have no good answer to a situation. The main draw of Antigua-types over other guns is that you can attack any time you want since you only need the tiniest opening, but both Roarmulus guns are still a huge improvement over swords in that respect and Biohazard also adds splash damage, insta-kill options, and Poison. Speaking of that, if you go that way then between the Polaris, Biohazard and Brandish you shouldn't be hurting for lack of a bomb. That's not to say they are a complete replacement for a Blast Bomb by any means, but they cut far into the marginal value of adding any damage-oriented bomb.

I think the relationship between the Roarmulus guns and the Alchemers is very isomorphic to that between Brandishes and Sealed Swords, except for Alchemers being relatively weak at low star levels. Roughly speaking, Cutter:[Brandish and Calibur]:Sealed::Antigua:[Pulsar and Catalyser]:Alchemers. (or, alternately, Cutter:Antigua::[3-swingers]:Roarmulus guns::Sealed Swords:Alchemers) The analogy also breaks down significantly on the Cutter vs. Antigua comparison due to damage type options being different, though, but otherwise it should give you a good idea of what you will like within that set if you have also tried either Antigua or Cutter. My top four weapons right now (in no particular order) are Virulent Catalyser, Gigawatt Pulsar, Blazebrand and Dark Thorn Blade, so the similarity to your favourites suggests you might also find similar results. Your Freeze also might synergise better with Catalysers in general than my Fire does, since sticking charge globs does not unfreeze targets and Freeze might make the globs easier to hit with.