If you're talking about the flash player then it's adware, so while not necessarily dangerous directly it can be dangerous indirectly and very annoying.
Basically if you download it it'll bombard you with ads, and the ads that it sends you can certainly contain malware. To top it off, lightspark can be difficult to remove, and if it's constantly annoying you then it can definitely feel like a virus.
An adblocker may stop the bad ads from happening but it'll still probably affect your processing power while using as it's constantly trying to send you ads and constantly getting blocked, so it bloats your PC and browser experience.
To top it off, flash itself is inherently dangerous now, it hasn't been supported for a very long time and effectively has become a wild west, so be very careful using flash tools.
If you really wanna play flash maybe use Ruffle, it's open source and free so everyone knows what the code is and what it does.