Ye Olde Poisonous Poison is massively helpful in killing Mary Sues. The PPC generally likes it because it is a fanwritten creation; if reality-type stuff is more to your taste, try curare. Ye Olde Poisonous Poison's biggest moment of fame was in the first death of Maeluiwen in "The Most Interesting Adventures of Maeluiwen," when Gríma shot her with a poisoned dart for sleeping with Éowyn. Since its introduction, it has become a very popular way of disposing of unwanted characters, and, occasionally, fanwriters.
Ye Olde Poisonous Poison is created whenever an author has his or her characters use an undescribed "poison" in a fic. Unlike normal poison, which only works quickly if injected, YOPP works quickly and dramatically under all circumstances. Badfic authors tend to be horrible at toxicology... a fact that the PPC takes advantage of by weaponizing YOPP.