Ordinarily, Adrian is not able to summon the Gate of the Underworld unless there are specific conditions. It is actually very strict in summoning this gate as it practically brings a god of death from their death realm to the mortal realm without any sacrifices needed. Fortunately for Adrian, the conditions are already sufficient.
The first condition is that a Taboo has been broken in the area which is done since dead beings were called upon from the underworld. The second condition is that the area must be able to handle the divine body of the god of death which is also true since the current Asgard is a divine realm. The third condition is that the souls of the dead had nowhere to go which is also true since Adrian could see all the dead souls just floating around without a place to go.
