It is menadione, a synthetic form of Vitamin K (vitamin K3) It also goes by several names in ingredient lists:
Menadione sodium bisulfate
Menadione sodium bisulfite
Menadione dimethylprimidinol sulfate
Menadione dimethylprimidinol sulfite
Menadione dimethylprimidinol bisulfite
Dimethylprimidinol sulfate
Dimethylprimidinol sulfite
Dimethylprimidinol bisulfate
Vitamin K supplement
Menadione supplements for humans have been banned in the US.
The Troubling Controversy Over Menadione in Dog Food The Dog Food Project - Menadione (Vitamin K3)
Some pretty surprising brands contain menadione like Newman' Own, Rachel's Ray's Nutrish, and Life's Abundance.
Dog Food Comparison Chart - Dog Food Ratings of Premium Dog Food and Supermarket http://www.mynaturalsite.com/dogfood.php