Frank J. Wilstach, comp. A Dictionary of Similes. 1916.

Pitiless

Pitiless as a Maxim gun.
—Anonymous

Pitiless as northern night.
—J. W. Barber

Pitiless as night.
—Ambrose Bierce

Pitiless as the grave.
—Gerald Massey

Pitiless as driving sleet.
—John. C. Neihardt

As pitiless to them as the Hyrcanian tiger to a lamb chop in the original wool.
—New York Sun

Pitiless as the strokes of an iron hammer.
—Ouida

Pitiless as hail from heaven.
—C. C. Fraser-Tytler