Slack is a communication app similar to whatsapp, telegarm but has some advance features like channels, integration with applications like gdrive etc.
Slack has its own APIs which can help us to use slack as c2 server.
Blog Link:- https://3xpl01tc0d3r.blogspot.com/2018/06/how-to-use-slack-as-c2-sever.html
Starts at Saturday June 09 2018, 01:10 PM. The sessions runs for about 1 hour.