Arcane Containers can only be opened by someone on the proper step of the Karazhan attunement quest chain. Originally, the key fragments were in the container. However, stealth classes found that they could stealth all the way to the container and take the fragment without needing to fight anything. In order to combat this, once the container is opened a Fragment Guardian (level 70 elite) will spawn. Once the guardian is defeated, the key fragment can be found on its corpse.
The first Arcane Container can be found to the left side of Murmur in the Shadow Labyrinth. In order to open the container, someone must have the quest  . This means that the party must clear almost the entire dungeon to get the . While it is possible to open the container before fighting Murmur, most groups wait until Murmur is defeated.
The second Arcane Container can be found in the water to the right of the path leading to Hydromancer Thespia. You do not need to defeat Thespia in order to get the . In order to open the container, someone must have the quest  . The container is down at the bottom of the pool so there is a risk of drowning if the fight takes too long.
The third Arcane Container can be found in the room containing the Negaton Screamers and the Negaton Warp-Masters. This room is a short distance past the first boss, Zereketh the Unbound. You do not have to defeat Zereketh in order to get to the . In order to open the container, someone must have the quest  .