Damage at 30% Hearts or Less is a weapon seal in Hyrule Warriors: Age of Calamity. It increases the damage dealt when the character's health is below 30% of the character's current maximum health.
The maximum total damage buff from this seal is capped at 50%, so a pair of tier 3 seals (double plus sign) and one tier 2 seal (single plus sign) will give the highest possible benefit.
Note: A weapon receives a +5 damage buff from having any two weapon seals of the same suit, a +15 damage buff from having any four seals of the same suit, and a total +20 damage buff for having stacks of four seals in one suit and two seals in a different suit. Six seals of the same suit will only give a +15 damage buff. Therefore, if you wish to plan your weapon build in advance, it is important to know the hidden seals of your chosen weapon.