Litotes
A form of understatement that affirms something by denying its opposite.
Litotes makes a point through negation, as in “not bad” for good or “no small feat” for a large achievement. Writers use it for modesty, dry humor, or ironic emphasis, letting restraint do the persuading. The deliberate downplaying often signals more than a direct statement would.
Example
The poem’s habit of calling a fierce clash “no pleasant meeting” understates carnage in a way that heightens its grimness.
Beowulf · Anonymous (Old English poet)