The Last Agni Kai
Target creature you control fights target creature an opponent controls. If the creature the opponent controls is dealt excess damage this way, add that much {R}. Until end of turn, you don't lose unspent red mana as steps and phases end.
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Official rulings (2) · Scryfall
In some cases, the creature you control that is fighting may have deathtouch. Even 1 damage dealt to a creature from a source with deathtouch is considered lethal damage, so any amount greater than that will cause excess damage to be dealt, even if the total amount of damage isn't greater than the opponent's creature's toughness.
Excess damage has been dealt to a creature if the damage dealt to it is greater than lethal damage. Usually, this means damage greater than its toughness, although damage already marked on the creature is taken into account.