Alas Leviathan

The phrase in that day is very common in Isaiah as I pointed out here. That day seems to include the end of Leviathan.

In that day Yahweh will visit with his sword, the intransigent, and the great, and the courageous, against Leviathan, the snake that runs away, and against Leviathan, the snake of crooked ways,
and he will slay the dragon that is in the sea.

To day Isaiah 27 is in view at 929, the rejuvenated exploration of the Hebrew Scriptures out of Israel.

Isaiah 27:1 Visiting Leviathan