Circle of Flame

• WWizards of the Coast2014-07-18
  Official Ruling
  Circle of Flame's ability triggers only if a creature without flying is declared as an attacker during the declare attackers step. If a creature with flying attacks you or a planeswalker you control and then loses flying, or if a creature without flying is put onto the battlefield attacking you or a planeswalker you control, the ability won't trigger.

Rulings via Scryfall

• WO
  Wizards of the Coast2018-07-02

  Circle of Flame was banned in pauper on July 2, 2018. Circle of Flame will be banned in Pauper to correct a format issue where it is currently legal in the format despite only appearing at common as promotional cards. Source: https://magic.wizards.com/en/news/news/january-21-2019-banned-and-restricted-announcement