Huatli, the Sun's Heart

Each creature you control assigns combat damage equal to its toughness rather than its power.
[−3]: You gain life equal to the greatest toughness among creatures you control.

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Huatli’s first ability doesn’t actually change any creature’s power. It changes only the amount of combat damage it assigns. All other rules and effects that check power or toughness use the real values. For example, Domri’s Ambush won’t cause a creature to deal damage equal to its toughness.

The greatest toughness among creatures you control is determined only as Huatli’s loyalty ability begins to resolve.