First you will need to make sure you have Keitha unlocked and load into the Lake of Foscari map. The "Seal" is located in the top right of map (
https://progameguides.com/wp-content/uploads/2023/04/vampire...) and will be marked with a question mark if you have the Milky Way Map. Go to the seal and evolve your Flash Arrow (do this by having a max level Flash arrow, Bracer and Lucky Clove at the same time). Now stand near and aim at the seal to break it and unlock the achievement.
*Credit to Progameguides for the map
Edit: A number of people have mentioned issues with delayed/failed unlocks. It seems the best solution at the minute is closing the game and loading it back up.