After Merlin was warned by Emilia Diablos that the cult of Moloch is in grave danger he decided to act.
Merlin destroyed the Sending Stone that he used to communicate with Emilia and escaped his hideout in Mt. Dread, leaving Khan there unguarded. His goal was to start a summoning of a fiends in Lodbaw to distract those who look for him and hopefully kill them, while in the meantime he will be able to interrogate Khan and reveal the secret way of summoning Moloch.
Merlin traveled north, to Gath, he wanted to manipulate some Giants to join his forces showing them a familiar face. The Giants of Gath didn't fall for his tricks and chased him away from the frozen city. When he tried to recruit them, Merlin revealed a plan to summon Fiends from the abyss to attack Meadheavan.
The Giants have a history of almost losing Gath to Fiends during the Death War, they felt the need to prevent the summoning of more Fiends and wanted to prevent the attack on Meadheavan so they sent 4 Cloud Giants to control the weather and chill the entire area, effectively killing the Fiends as they spawn. But the storm they created was halted by Shrok'lakelev who where in the middle of one of the hurricanes that spun around Meadheavan.
Shrok'lakelev were tossed by the storm over the river and into Quora, where they saw the Cloud Giants and assumed they want to destroy Meadheavan, they immediately attacked and stopped the concentration of the Giants, effectively stopping the storm.
The pause in the storm gave a short window of opportunity for Fiends to pour out of the summoning portals that Merlin created, a swarm of Fiends came out of those portals and rushed towards Meadheavan.