Hello dear.
I think you are talking about the achievement Prospectre. You can get it after kill the Boss "THE THAIAN"
The Thaian is the new boss that you can find/kill during the "Feaster of Souls Quest".
As you said the achievement "Prospect" was implemented with the recent Winter Update. In order to obtain this achievement you will need start the quest Feaster of Souls Quest which requires access to Lion's Rock and Sea of Light Quest completed. In the middle of the quest you will need to defeat the boss Thaian, killing him you will receive the achievement "Prospect".
This achievement gives you 1 point.
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