Complex sentence: During the time Moonstar was part of the team, Forearm ends up battling forces from the other-dimensional realm of Asgard, which had appeared due to Moonstar's history with the realm.
Split into simpler sentences: During the time Moonstar was part of the team, Forearm ends up battling forces from the other-dimensional realm of Asgard. They had appeared due to Moonstar's history with the realm.