Locate a summoning residuum within Miragul's Phylactery: The Anathema and use the crystal-tipped rod on 5 of them to absorb it. They are semi-random spawns but seem to reliably appear at the following locations as well as random spawn points throughout the zone:
As well as getting updates from the locations listed above, you can spawn a summoning residuum by killing a hostile malevolence, a dark malevolence or an envious malevolence. However, this is a random event (approximately one residuum for every two kills) and only one residuum will pop at a time: you need to use the crystal-tipped rod on the residuum before killing the next mob if you want another one to pop.
