When activated an Entropy Arch becomes a bridge between wherever it was opened and Chaos. The first of these were made in the Dawn War by the primordials. The distance from the core of the universe was simply too far from distant systems, so entropy arches were made to make stable rifts; and perhaps most importantly, rifts made by entropy arches are undetectable by weave spiders.
Those faraway systems are the playgrounds of the Creationists. Out there, they are creating worlds and seeding them with life. They bring order to chaos. This needs to stop now, before those seeds become their armies.
An entropy arch is a magical device of legendary artifact-like power. The main ingredient for making one are entropy fragments. The construction of one of these rift-making devices has always lead to terrible conflicts. In the Dawn War, the gods and primordials fought across Bal-Kriav and other worlds with the primordials using entropy arches at places like Regadnîn and Tâchan. When the arches were opened, primordial armies poured out of them, warring with Covenant forces. One of the most used entropy arches was one at the bottom of Piranoth Cyst. This one was destroyed by then angel Naraz-Nâru. He used three items to do this, relics collectively called Sig Mak Gan.
... the opening of these chaos arches creates a gateway between Chaos and a realm. Raw chaos floods through it like a thousand tornadoes. This wave of energy is so powerful that nothing is thought to be able to survive it. It can shear the landscape, turning vast areas into motes of their former selves. A lake may float upwards or sink into the ground as if it were immensely heavy, earth may float skyward, or be changed to dust and lost on the wind.
When an entropy arch is first opened, a blast of entropic energy pours forth. The source of this blast wave comes from the rift's other side, the realm of Chaos. The best documented event of this happening occurred on 7 Witchrite 1832. On this day, an Entropy Arch was dropped into the heart of Sapthiladân. This descent into a hail of cannon fire, magic, and arrows was done by the Pillar of Horns. This enormous voidship dropped the Entropy Arch into the central square of the city. The Arch then activated and opened its door to Chaos. The opening of the arch caused a large swathe of the city to be reduced to rubble. The blast wave continued past the city where it then struck the Kerthjorg highlands. The chaotic energies sheared a string of mountains like a hot knife cutting through butter. The result of this event was the creation of the Skyldr-Kerthjorg sky islands.
The entropy arch used at Sapthiladân was built with entropy fragments removed from the Pinnacle of Chaos. A few hours after the rift's opening, the vanguard of the 3rd Horde of Chaos, under Drachmon, began to arrive on the world Bal-Kriav. This army added their numbers to the Flux Pact at war with the Khazarar Empire in the Cinazan Front.
In the Dawn War, primordials preferred using Entropy Arches over creating rifts. This is because the latter are tears in the Web of Magic, which unerringly attracts the attention of the Web's indomitable guardians - the weave spiders.