A hecatomb is a great sacrifice of cattle in honour of a specific god. Although the name signifies the killing of a hundred beasts (the name comes from ‘hekaton,’ meaning one hundred). In practice the ritual slaughter of, as few as twelve animals was sometimes considered a proper offering.
Hecatombs can be performed in game by first accessing the god of your choice in the Divine Will panel, then pressing the hecatomb button. The great sacrifice will be performed instantly by the faction's priests for the selected god, gaining the player's faction a large amount of divine favour in exchange for the specified resources.
Hecatombs can only be performed for a single god at a time, and the required cooldown time must be passed before another can be performed.