Eclipse's main purpose is to strengthen the player's Myth Units, making them more efficient for their cost. This is especially useful in the early game when resources, especially Favor, is limited and few Myth Units can be trained. Even so, players are encouraged to train as many Myth Units as they can to maximize the benefits of this god power. A group of Sphinxes along with some Scarabs powered by Eclipse can cause severe damage to an enemy town. Isis worshipers can also pair Eclipse with Ancestors to make their Minions much stronger for the limited time they will be around. More patient players can save this God Power to boost their Titan which otherwise the Egyptians cannot improve in any way. Unlike Bronze, newly trained Myth Units will also be strengthened by Eclipse, so players are free to queue up their Temples.
Eclipse has the added effect being a God Power shield. It lasts for just over a minute, essentially shutting out the other player's God Powers for a fairly long time. This prevents enemies from defending themselves with Ceasefire or can even delay the use of an offensive God Power such as Earthquake. The player's own God Powers cannot be cast during Eclipse either, so they must make sure to cast the aforementioned Ancestors first or not hinder an ally's own attempt at attacking.
Eclipse is one of four global God Power shields, the other three being Rain, Ceasefire and Fimbulwinter.
According to pre-release footage from German gaming magazine Gamestar, Eclipse was originally going to lower the LOS of Buildings and non-mythological units to very small amounts (about 2-5 feet) the reason for the absence in the final release is unknown.
Also, according to the Age of Mythology handbook, Eclipse originally had the effect of reducing the attack range of Archer and Siege units by an undetermined amount.