Hey! Was having trouble finding that dark knight minigame/job as well as if there was anywhere I could find some to fight? they seem to have disappeared!

The mini game happens literally at the end of the current build. It's the job the twins give you.
And they appear for me during night encounters at various locations. They appear randomly though. You can still fight them at the Library when you book a room.

Bjeorr

Go to the Castle Gate during Day and Evening to talk to the Twins for the Dark Knight Job. (The Job where you fire Ballistas at them for Diamonds)

During Mornings (i'm guessing the Twins have the late shift), there are other Guards there at the Gate, Renny and Stimper.

You can also Duel them via Library if you Rent a Room.

Dark Knights usually appear at the certain Jobs during Night, while cutting wood, mining etc

There are different types of Dark Knights depending on location as well, Top, Bottom etc