Depending on your setting and desired outcome for the poisoner, uraninite (aka pitchblende) might be an option. It has historical uses in glass making and pottery glazing, which could provide justification for why someone would have it.

It contains Uranium, which is radioactive, but I don’t believe will bioaccumulate, but can build up on surfaces, tools, and clothing providing a source of long-term radiation exposure. In addition, it contains lead, which does bioaccumulate, providing a source of gradual long term poisoning as well as radium which also bioaccumulates and is radioactive, providing an additional source of longterm radiation exposure.

We have twink primarchs already. Fulgrim on the Chaos side and Sanguinius on the loyalist side.

“I’ll have you know there’s no pusseeee!”