Just renewing my idea from last year for St. Patrick's Day...

The Leper Khans

Lords of all genetic warfare, these beings are despised is every quadrant of the Universe, while their wares are quietly purchased and stored for future usage.

They drop:
Improved Engineered Viruses (prevent planet growth for 24 hours and reduce population by 25%)
Targeted Biovapourizers (kills 2/3rds of population and adds planetary effect: Devastated Landscape)

Maybe other stuff instead, just throwing the idea out there to see what becomes of it