"Which do you think is a better deterrent: a moat of water or one of fire?"
—Chandra Nalaar

Circle of Flame

Whenever a creature without flying attacks you or a planeswalker you control, Circle of Flame deals 1 damage to that creature.

Magic 2015

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Magic 2012

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.

Circle of Flame is banned in Pauper (not just non-legal), due to a special case on MTGO: https://magic.wizards.com/en/articles/archive/news/january-21-2019-banned-and-restricted-announcement