The news of the arrival of Horror's horde affected the two civilizations in different ways.
The cultivators, being the main target of Horror's nature, were more wary of the threat.. While the elementals saw the imminent invasion as something not unlike any that preceded it. Horror's domain wanted their territory, just like the cultivators before them, and they would either escape, or fight it.
Nevertheless, a threat was a threat, and Horror's arrival was a fight they had no much hope of winning. For that reason, after many meetings, the Elemental army had eventually decided to put aside their differences and sign a temporary truce with the Immortal Armada.
Despite this agreement, the elementals did not let go of their hostility. After all, it was easy for the cultivators to forget their greed in times of crisis. They knew that the moment the horde's threat was successfully repelled, the cultivators' true nature would re-emerge, and their fight would resume.