There are many applications for this God Power. If the player has spread their forces too thin, the Underworld Passage allows armies to be transported quickly to wherever they are needed. It can also be used to bypass an enemy's defenses and attack them where they're vulnerable. However, one should be careful when placing it since if one of the portals are destroyed the power is negated.
On maps like Archipelago, Team Migration, River Nile and Islands, the Underworld Passage could become a very efficient and secure way to transport units between two islands or crossing the river, thus negating the need for Transport Ships. In maps like Jotunheim, where the map is divided by mountains and where there are only narrow chokepoints form where one can invade each other's team, Underworld Passage can be used for a sneak attack.
It should be noted that Titans cannot pass through the Underworld Passage. Also, depending on the map and placement of each end of the Underworld Passage, Azure Dragons and War Salamanders can either choose to go into the Underworld Passage or bypass it altogether due to being amphibious myth units. If the Azure Dragons and War Salamanders are made to cross the body of water instead, the player will have more room for units to transport through the Underworld Passage.