Where is this art from?

I didn't know what forum to ask this in but here goes.

Are all of these pieces of art from the same book or encyclopedia? If so what is it called?

 

Hmm I don't know, have you tried a reverse image search? Go to like imagesgoogle.Com or something and upload the pictures?

 

The Complete Star Wars Encyclopaedia:

https://starwars.fandom.com/wiki/The_Complete_Star_Wars_Encyclopedia

apparently does have the first official depiction of Darth Millennial

https://starwars.fandom.com/wiki/Darth_Millennial

In The Complete Star Wars Encyclopedia, Millennial's name is misspelled as "Darth Millenial." He also received his first official depiction in that work, which showed him to be Human, canonizing Peña's previous comments that he was likely a member of that species.[8]

so that one's probably from there.

The Bane pic definitely comes from The Essential Atlas - I looked. I'm not sure where the Darth Tenebrous pic comes from - possible Star Wars Insider's The Tenebrous Way short story?

 

If you go to each character's Wookieepedia article and then find those images (which are likely there in the article somewhere, if not the main pic), click on them and in the box that pops up showing the image, look for the little question mark in a circle in the upper right hand that says "more info" and hover over it. That should give you the source for the pic.

Obviously only works if the image is there though I'm 99% sure I've seen at least that Bane & Millennial pictures there.