Gilgamesh is a character defined by his pride. Armed with the weapons of heroes that came after him, he was able to live until the end of both Fate/Zero and Fate/stay night: Unlimited Blade Works. One of his many armaments is the sword Ea, along with his unique Noble Phantasm called Enuma Elish. Here's why Enuma Elish is something you don't want to mess with.
Ea's Origin
Ea, and the ability Gilgamesh performs with this sword, have their roots in Mesopotamian mythology. Ea is the deity for water and wisdom who plays a role in the Mesopotamian creation myth. After Marduk defeats Tiamat's champion in the myth, they consult with Ea to create humanity. As a result, humans were made using the remains of the god that started conflict with Tiamat.
Meanwhile, Enuma Elish refers to the Mesopotamian creation myth itself. It signifies the death of what once was and the birth of a whole new world. Considering the abilities of this sword, this name matches the destructive power that it could unleash on the world.
Ea's Appearances In Fate
Throughout most of the Fate franchise, Gilgamesh is known to rely on his Garden Of Babylon to deal with enemy Servants. In a sense, this was Gilgamesh holding back his true power due to his ego as the King Of Heroes. Only in the face of people that Gilgamesh considers worthy would Gilgamesh take out his sword Ea.
There are two major instances where Ea is ultimately used in Fate. The first opponent who earned Gilgamesh's recognition is Iskandar near the end of Fate/Zero. When arriving at the bridge where this fateful duel would happen, Iskandar was not at his fullest strength. Despite that, Gilgamesh notes that his aura is stronger then previous times. After a drink is shared between the two kings, they face off against each other in Iskandar's Reality Marble.
Gilgamesh is pushed to use the sword against Shirou in Fate/stay night: Unlimited Blade Works in the heat of the moment. Shirou couldn't defeat the King Of Heroes on his own, and it is only through the help of his friends that Shirou could fight against Gilgamesh in the end. Using Rin Tohsaka's magic crest and Archer's Unlimited Blade Works, Shirou is able to put up a bit more of a fight. Catching the King Of Heroes off-guard, he attempts to get Ea so that he can finish Shirou off. While the sword is materialized, Shirou is able to neutralize the threat before Enuma Elish could be unleashed.
Ea's Powers
Befitting its status as Gilgamesh's final trump card, Ea is hard to avoid once materialized. Using the special ability of the sword requires him to chant its name, but after he manages to do that the sword wipes away everything that stands in its way. Once the sword is activated and the rotating pieces of Ea start to whirl rapidly, it generates a strong wind current that becomes capable of tearing away at the fabric of space reality.
When hit with Enuma Elish, the target area is pulverized by the compressive force of Ea's attack. In fact, this power is so strong that it can create a void that swallows everything that once existed there. Whether it be damage from an oncoming Noble Phantasm or a Reality Marble, Ea can take everything into its void to leave behind a state of nothingness.
Gilgamesh's fight against Iskandar demonstrates how threatening Ea is. Within Iskandar's Reality Marble, Gilgamesh stands alone against Iskandar and his Ionian Hetairoi. With each of his men being considered on the level of spirits, Iskandar's men could have spent Gilgamesh's time draining his Gate Of Babylon. Deeming him worthy of Ea's power, Gilgamesh takes out this sword and uses Enuma Elish. The results of this attack were catastrophic for Iskandar's men.
Enuma Elish tore away at the fabric of Iskandar's Reality Marble. What was once a desert field becomes a fragmented chasm that all of Iskandar's subordinates fall into. While the King Of Conquerors manages to come out of his Noble Phantasm, the damage of Enuma Elish not only takes out this army but also reduces the desert plain to nothingness before it fades away.
This is not to say that Ea can't be prevented. The fact that Shirou manages to cut off Gilgamesh's arm as he takes out Ea goes to show that it may be possible to keep the sword from activating if you're quick enough. If an opponent can close the gap between them, this is the best way of handling the threat. However, once Enuma Elish has been activated, nothing can stop its wrath. This weapon solidifies Gilgamesh's status as one of the most powerful Servants in Fate's anime catalog.