Mortal damage is a status ailment in both EarthBound and Mother 3. It is the only status ailment that requires action within a certain period of real time, rather than depending upon the game's turn-based system.
Mortal damage in battle
Mortal damage occurs when a party member takes damage which would reduce their HP to 0. Because of the Rolling HP Meter, the character is not instantly knocked unconscious. If the affected character is healed before their HP reaches 0, they will have successfully avoided fainting until taking mortal damage again. If there is time, the character may also defend themselves while waiting to be healed, reducing the rate at which their HP drops. The odometer will stop rolling downward at the end of the battle regardless of damage taken. Only the characters in the player's party have odometers, and thus are the only possible victims of mortal damage. Teddy bears and the Flying Men in EarthBound do not have odometers, and will instantly die when they take mortal damage.
Healing and damage taken while the odometer is rolling is based on the number which the odometer is trying to reach. This means a heal of 100 on a mortally wounded character will make the odometer start going towards 100, regardless of where it was at the time. This works the opposite way too; if the character being healed to 100 is struck again and their HP is reduced to an amount lower than the odometer's current reading, then it will begin rolling downward again.
Mortal damage causes one other disadvantage: every time a character is mortally damaged, the screen shakes more than usual and the message "name received mortal damage!" appears. This causes a delay, allowing even more time for the victim's HP to drop.
If a party member with a high enough Guts stat takes mortal damage, there is a chance for their odometer to stop at 1 instead of 0 even if they are not healed.
Other occurrences of mortal damage
Despite being curable at a hospital and even by PSI Healing, most occurrences of mortal damage as part of the game's storyline are fatal.
- In EarthBound Beginnings, Teddy seems to take mortal damage from the fight against R7038, which severely incapacitates him for the remainder of the game. He is implied to die in the Famicom version, but it is clarified in the NES prototype and GBA re-release that he has fully recovered following Gyiyg's defeat.
- Later in Mother, EVE sustains mortal damage from the R7038XX fight, which causes her to self-destruct as a last resort, destroying both robots to an irreversable point.
- In EarthBound, Buzz Buzz seems to take mortal damage after being swatted by Lardna Minch. Buzz Buzz then fades out of the world after giving Ness the Sound Stone and a few words of advice.
- When Fassad falls from Thunder Tower, it could be considered that he took mortal damage upon touching down, as the result of his fall is him needing cybernetic repair in order to survive.
- Later in Mother 3, Claus also takes mortal damage when his supercharged thunderbolt is intentionally reflected back at him by Lucas's Franklin Badge, in the act of Claus's suicide.