en_tn/isa/27/01.md

On that day

"At that time"

Yahweh with his hard, great and fierce sword will punish

Yahweh having the power to destroy his enemies is spoken of as if he had strong, large sword. (See: rc://en/ta/man/translate/figs-metaphor)

the monster that is in the sea

This refers to Leviathan.