Or Slack-Archiver, whatever. Basically, set up outgoing hooks on Slack, an incoming hook to monitor for errors, and this, and all messages can be stored in a database.
On Ubuntu or Debian? You should be able to just:
git clone https://github.com/Guard13007/slackiver.git
cd slackiver
chmod +x ./install.sh
./install.shIf you can't or are using another OS, look at the script to see how it works. Also, note that the server will be running on port 9443, so you should set up a proxy to it if you want it on 443.