What is the Livyatan Feast?
The Livyatan Feast is mentioned many times in the Talmudic and Midrashic literature. Livyatan is a giant fish which was created during the Six days of Creation. G-d created one male livyatan and one female, which never reproduced because He "castrated the male and killed the female and preserved it for the righteous in the future."
The Midrash describes the battle between the Shor Habar (Wild Ox) and the livyatan. The shor habor will batter the livyatan with its horns and rend it, and the livyatan will batter the shor habar with its fins and pierce through it.
Although the livyatan banquet will indeed occur, its mysterious details reflect the essence of the new era. The livyatan lives in the sea, symbolizing the hidden spiritual worlds that exist in the revealed Divine infinity. In contrast, the shor habor dwells on land, symbolizing the visible lower world. Thus these two creatures represent the two major facets of serving G-d: The spiritual acts effected in the higher worlds through man's actions, and the sanctification and purification of the lower world.
Some Jews resemble the livyatan, aspiring to exalted spiritual heights rather than bringing holiness down to this world. For example, R. Shimon b. Yochai lived in a cave for thirteen years, practically divorced from the physical world; his divine service was spiritual. Other Jews resemble the shor habar, forgoing peak spiritually in order to sanctify the world and prepare it for Divine truth.
This symbolism explains why the livyatan will slaughter the shor habar and vice versa, for each attitude will "slaughter" the other in the ritual sense, i.e., they will render each other fit, with one perfecting the other. That is, those Jews who chose the livyatan approach to serving G-d will elevate the Jews who employed the shor habar method, and vice versa.
SOURCES: Talmud, Baba Batra 75a; Maharsah ibid.; Likkutei Torah Parshat Tzav.