Final answer:
The irony with the poisoned rapier (sword) is when the intended victim becomes the perpetrator's downfall. An example of this can be seen in Romeo and Juliet when Romeo uses a poisoned weapon to take his own life, only for Juliet to wake up and find him dead.
Step-by-step explanation:
The irony that happens with the poisoned rapier (sword) is when the person who intends to use the sword as a weapon ends up being harmed by it instead. This creates a situation where the intended victim becomes the perpetrator's downfall. An example of this irony can be seen in the play Romeo and Juliet, where Romeo uses a poisoned weapon to take his own life, only for Juliet to wake up and find him dead, leading her to take her own life as well.