On many occasions when this has occurred, it has very literally been done by random characters that were curious if it would work or not. Several times that it happened, perhaps lag, I found that I was completely alone in the area you are conjured. Unable to leave the Cell or the Pits to the point I had to ask other characters to break the immersion to come and free my character.
Whilst the practicality of the Slave Callers is clear, the ability to abuse is currently very unbalanced, either by disrupting RP or very literally allowing the ability to grief in the sense that any Slave can be conjured and killed without the ability to refuse it. [ Unless you in Werewolf form
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