Machine Duplication
I originally thought this card was crazy in the deck when it was first announced, since you could essentially turn brass+dupe into Galatea and then Summon Sorc/Longirsu. But since brass is still only good if you have a target in hand, and there is no card that puts orcusts in your hand, I don't personally think the play is that great. Now potentially milling 9 cards of card trooper is another story. Besides those cards the only other potentially target I could think of would be World Legacy - World Chalice for some cheeky Cyber Dragon Infinity access, but that seems very counter intuitive.

Run 0 or 3

Instant Fusion
IF can be good, especially going second since you can use Thousand eyes to steal a card, then normal Harp and get a full board out. But It has a lot of weaknesses. First, extra deck space is kind of valuable, and I think you want to run a second target(preferably a machine) for summon sorc access. The problem here is that the only DARK machine target would be Chimeratech Rampage Dragon, which can only be properly fusion summoned. So you either are locked to cards that can't get you sorc, or cards that can't be summoned after using Orcust effects. Final note, it is a one card answer to the evil that is Inspector Boarder. I haven't playtested it enough to see whether or not it's worth it, but the potential is certainly there.

Run 0-2

Called by the Grave Mandatory in this deck since your best card is vulnerable to ash.

Run 2

Twin Twisters
In a deck that loves to discard and in a format with lots of backrow, I think this can be worth maindecking. It feels bad in certain matchups, but losing to floodgates or backrow without any answers feels worse.

Run 2-3

Soul Charge It's free wins.

Run 1

Here's a quick list of other spells and traps that have synergy in the deck that you could use in your side or main deck.
- Sky Striker Maneuver Hornet Drones( w/ Kagari+Hayate in Extra)
-Limiter Removal
- Mistake
- Gozen Match
- Virus Cards

Other LINKS

Summon Sorceress If you're like me you sold your copy as soon as you heard about the reprint, only to buy another one for a higher price because you wanted to play orcusts and this card is mandatory. I don't care if it's expensive, you need this card to play the deck. I don't know how our OCG brethren are coping without her, but she is fundamental for the bread and butter combo. You don't have to pay a ton of attention to where you place her, but I use the right extra monster zone for consistency's sake.

You have to run at least 1

Topologic Bomber Dragon This decks actual boss monster. I've found that the effect is either completely devastating, or useless. You should be able to reliably use the nuke effect at least twice on your opponent's turn. I usually try to activate it after I see my opponent get out two monsters, since at that point they could just knock out babel with Phoenix(Cerberus is pretty weak against this deck). Don't forget that his effect will activate for your opponent's linked zones as well. Many decks that can't immediately answer Topologic will be limited to one extra deck monster until they deal with it. Which is easy pickings for Longirsu. Don't forget about the burn damage effect, it burns for original attack, meaning that you can shrink something with Orcustrion, and then hit it with Topologic for enormous damage.
Run 1

These two and the Orchust links are what I would consider mandatory for any Orchust deck. The rest is up to your discretion but I will list some of your stronger options here.

Borreload Dragon Not super hard to make in this deck and one of the best DARK link monsters. Provides removal and tricky for many decks to deal with. Also has side-ways link arrows which can be a blessing in the deck for getting out more link summons.
Borrelsword Dragon
Another really strong DARK link, great for enormous damage and closing out games. In the mid-game especially tossing out an orcust in attack position can make it easy to use his double attack effect. Pretty decent arrows as well.

Triple Burst Dragon
Pretty awesome card overall in this deck, many people are using this to answer Rei in sky striker decks, or other cards that float after battle. You can tag out to bring back galatea which is nice, and the piercing damage is a cool bonus.

Knightmare Package( Unicorn, Phoenix, Cerberus, Mermaid, Gryphon)
Unicorn fits perfectly into the deck since it's a dark, and putting it above Longrisu/Orchustrion will let you replace the card you discarded. Phoenix and Cerberus are trickier since you can't summon them after using an orchust effect. They both help quite a bit though and can easily be made with cards summoned on your opponent's turn. Note that cerberus summoned underneath Topologic will be co-linked and not destroyed by Topologic's effect. Mermaid can be good if you're afraid of Iblee stopping your Danger! cards. Skip Gryphon. Pour one out for Goblin.

Topologic Trisbanea
I forgot that run this card, but it's good situationally. Banishing your own monsters isn't that detrimental, and wiping out a field of backrow can win you games. Note that if you try to use this card like bomber dragon however, you will banish your copy of Babel. This isn't the end of the world, and often the right play to make, but just be aware of it.

Linkuriboh
You need this to make brass work if you draw it. You can also potentially use it's effect to get additional nukes from Topologic.

Wee Witch's Apprentice

Good if you're finding yourself often with two non-orcust cards on the field that you want to link away. Her extra 500 attack is nice since it makes Orcustrion enormous and Longrisu much harder to deal with. I rarely use her but it's a nice option and cool in the Danger Build. Be wary that if she is in the EMZ you may have trouble reaching Summon Sorceress

Akashic Magician
The best card in the game art wise, don't @ me. She's kind of meh in this deck since she's in many ways a weaker version of Longrisu, but has some good synergy with resetting Danger! cards or Kaijus. Same problem as Wee Witch though, can lock you out of Summon Sorc.

Topologic Gumblar Dragon
Hypothetically you could just go into Gumblar instead of Bomber off your normal combo and rip two cards from your opponent, and then a third on their turn. I don't think it's worth it since you don't have great options for filling your hand, but if you play a big Danger! engine it could be an option. His arrows also make it harder to keep your cards linked while reaching your opponent's EMZ.

Decode Talker I would skip Decode, Longrisu is almost always better, though I guess the protection from targetting could hypothetically protect Babel.

Beelze of the Diabolic Dragons
I don't run synchros but Brass Bombard is tuner. Nessie+ Brass could get Beelze out and if that sounds like a good play go for it.

Masked HERO Dark Law
This would be a pretty spicy tech if you are up for it. Many of your cards don't mind being discarded, so Mask Change II could potentially set you up for an alternate strategy. Just note that it conflicts with Topologic Bomber Dragon.