Experts in subtlety and poison, members wield toxins potent enough to quickly dispatch even the toughest of prey. Their rigorously trained and genetically-enhanced Assassins were selected from the finest alchemists that were capable of creating exotic concoctions that cause death without leaving a metabolic trace. They were said to have had the greatest level of success during the Horus Heresy.

https://wh40k.lexicanum.com/wiki/Venenum_Temple
venenum assassins