To my knowledge, there are no real stars that are "bad" stars. There are useful stars (used to navigate) and all the other ones.
Before the advent of telescopes, any of those tiny lights in the sky would have been seen as "stars" so it may not matter if it's a planet or comet or a star so much. We draw a distinction between those things that this fantasy society may not have. What I'm getting at is your search for "evil" stars may be limited by what we think of as stars.
As example, Mars. Even in ancient times, with the naked eye you could tell it was odd. It was red and it moved a lot more than most of the other little lights in the sky. It would show up in the night sky for a span of nights and vanish again, not to be seen for many more nights. When compared to other stars, it was magical in a way. But, at the same time, it is there enough that no long standing correlation can be made with evil events.
To be called a herald or a portent or ill it has to be very strange and not much seen and then have some bad stuff just happen the last time it (or something like it) showed up. So the more common foreboding things are comets and eclipses (both solar and lunar). But there is something else that you may like.
On August 6 in 1181 CE a whole bunch of people saw a light in the night sky bigger and brighter than any other star. Many of them feared it and said it was a bad omen. It wasn't a star, it was a supernova. It was there for six months. Yeah, people freaked out. After all, it could only be the end of the world. All that and they didn't know about the one in 1054 CE that remained in the sky for two years! We know of another half dozen supernovas seen in ancient times. Generally speaking, some people freaked out each time.
Again, we draw a distinction that the denizens of your D&D world may not. We know those were supernova. To them it's a thing that no one alive (at least no human) has seen before, hence it can't be good. Expand your definition of star to include these supernova and you can have something to reference. Otherwise, I don't think you will have much luck.