The chrysalun chambers are a draconic artifact that was a pocket universe in itself, designed for varied purposes depending on which dragonflight had created them. But first and foremost, it was used for imprisoning magical creatures by drawing on the prisoner's own magic to keep the latter imprisoned. However, the chrysalun chambers had a few weaknesses. Firstly, they required a lot of effort to create and use, so, there were few of these around. Even then, many of the chambers were unstable and flawed. After Neltharion's betrayal most of the Black dragonflight's Chrysalun Chambers were sealed away by the other dragonflights.
Lady Sinestra used hers to capture a nether dragon flying around the Dark Portal for her experiments. She later used it to trap Krasus in it to keep him from meddling with her plans and also for storage before she uses his energy to create more twilight dragons.