Rule 120.4a

First, if an effect that’s causing damage to be dealt states that excess damage that would be dealt to a permanent is dealt to another permanent or player instead, the damage event is modified accordingly. If the first permanent is a creature, the excess damage is the amount of damage in excess of what would be lethal damage, taking into account damage already marked on the creature and damage from other sources that would be dealt at the same time. (See rule 120.6.) Any amount of damage greater than 1 is excess damage if the source dealing that damage to a creature has deathtouch. (See rule 702.2.) If the first permanent is a planeswalker, the excess damage is the amount of damage in excess of that planeswalker’s loyalty, taking into account damage from other sources that would be dealt at the same time. If the first permanent is both a creature and a planeswalker, the excess damage is the greater of those two amounts.

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