Tonight's issue is over and Discord is working normally again. We're very sorry for the interruption this caused.
The root cause is under investigation. The team is actively working to understand the chain of events that led to this (and the last few) outages. We've got some good data on this crash and are putting together hypotheses that we can test in our staging & load test environments.
As mentioned in the last update, we do know the triggering event: a crash of one of our very large guilds that triggered a cascading failure in our sessions cluster. The crash is well understood and will not happen again, so we're confident we won't see a repeat of this cascading failure at this time.