The Cleansing Poison is a poison that has been created by Xiao Yi Xian to make Xiao Yan, "immune" to all sorts of poison. It is unknown how it is given, or how it is made, since we only see a short explanation by Xiao Yi Xian in chapter 69. She made it to be a "one time" thing, to cure any poison, it was wasted on a simple "5 sense" nullification poison. But Xiao Yi Xian noted that whenever a poison is ingested by Xiao Yan that might be lethal, a illusion of her will appear and she will help him cure it. The illusion had the exact same personality as the original Xiao Yi Xian, it can be comparable to a A.I. Later in the series Xiao Yan notes that "Your Poison won´t affect me thanks to Xiao Yi Xian." So this might indicate that it wasn´t a "one-time" thing. But since this goes against what the creator of the cleansing poison says, we cannot be sure, or it might simply be him being overly arrogant, due to knowing that she will help him if he gets a lethal poison.
Since this is the first time it has been seen, heard of and used. We cannot say that we know a lot about it. But what we do know is that it cures any poison ONCE, and that it helps him cure any poison. It is unknown if the Illusion´s help is limited by Xiao Yi Xian´s knowledge of poison, and/or if it has a connection to the original Xiao Yi Xian. You may not understand what I mean with connection, so I´ll explain. Basically what we know is that there´s some sort of "intelligence" based life form in Xiao Yan, but what we don´t know if this INTELLIGENCE is updated to what Xiao Yi Xian knows or not. It might require contact with the ORIGINAL, to be updated and so on.