Heroes of Ragnarok are similar to the regular Norse heroes, Hersirs, in that they are infantry units with a large attack bonus against myth units. Ragnarok is often used to exterminate a player rapidly. In addition, the Heroes of Ragnarok, while powerful against myth units, are not as effective against human soldiers. So unless the enemy is going heavy on myth units or has few soldiers it should not be used.
However, there is a high cost, as the player's economy will be greatly interrupted since it turns all worker type units into Heroes. It may be wise to invest in numerous Fishing Ships (on water maps) and Ox Caravans before invoking so that the player can still gather Food and Gold, respectively.
In Norse mythology, Ragnarok was believed to be the end of the world when the gods and the giants would fight a long bloody battle over the Earth.