I can't speak to poisonings but the word "Assassin" entered common usage by virtue of a very real group of Nizari Ismaili Shia who carried out the high-profile political murders that their name is now associated with. The death of Nizam al-Mulk is the most consequential of their murders that I'm familiar with; as grand vizier he was probably the most powerful man in the Seljuk Empire and he'd been incredible effective in his position for almost three decades. Their attempt to assassinate members of the Mongol leadership in the 13th century would be their downfall as the Mongols sought out and destroyed the mountain castles that had been their base of operations.

Perhaps as a somewhat amusing coda for an organization that is synonymous with political violence, the Nizari Ismaili Shia today are better known as the followers of the Aga Khan, the 49th Imam of their sect, who uses his position to promote peace and tolerance through extensive philanthropic work as well as through their particular interpretations and development of Islamic law and teachings.

I haven't read it but as far as I'm aware the standard work on this is Bernard Lewis' The Assassins: A Radical Sect in Islam.