Here's why this is an allusion:
* "Death-adder" is not a specific species of snake. It's a general term used for various venomous snakes, often associated with fear and danger.
* Kipling's use of this term draws on the reader's pre-existing knowledge of deadly snakes, creating an immediate sense of danger and threat from Nagaina.
* The allusion heightens the story's impact, making Nagaina seem even more menacing and emphasizing the seriousness of the conflict between Rikki-tikki and the cobras.
This is just one example, and there are likely other allusions within the story depending on how you interpret it.