Due to the amount of dwarves who died in the great defence, those who survived started wishing that they could say goodbye one last time.
In order to do so, a few magic users and herbalists come up with an idea to commune with the souls of the dead.
It took them a few months before they had a way to establish a link between the souls that wished to communicate and it took a few months more before the drug that the herbalists made as part of the process was ready to be mass produced.
The drug itself can create a connection with one or multiple souls, but it will not work properly if the participant cannot concentrate on the people to commune with. Those that are not able to meditate can use other drugs to reach the point, but it is well established that the communication drug can cause serious side effects if mixed with other drugs.
The side effects can range from minor skin irritation while it is working, to a painful death while being unable to breathe. Thus far it is not known why the reaction is so different depending on the second drug.
Doses of the drug is distributed among the population and then ingested.
In order to communicate with a soul the people you want to communicate with need to be clear in ones mind. It is for this reason that meditation is practised in advance.
Civilians are usually only participating if they've lost a loved one in the past year, as it's considered rude to disturb the resting soul more then once.
The rites are observed yearly on the day that the last enemy was slain.