If you are looking for general gameplay optimization, these highly-rated resources cover the most important current mechanics:
It is possible you are looking for information on one of the following similarly named or related topics: Selenium for Arcane Odyssey
A common high-level mechanic used to detect players or entities from a distance. If you are looking for general gameplay optimization,
You might be thinking of a specific character or place within the Bronze Sea or Nimbus Sea updates. Selenium for Arcane Odyssey