Aggro notification library.
Please note that LibBanzai-2.0 is mostly not needed any more. Blizzard provides some threat API and events that cover most of its functionality. However; LibBanzai-2.0 does not rely on threat, but rather on targetting information. The Blizzard events will not tell you if a boss targets a random member of your raid to apply a debuff or cast a spell on them, but LibBanzai will.
Most addons have transitioned away from LibBanzai-2.0 in the times since Blizzard introduced their events and API, but I believe it's still useful, despite the low CPU cost to keep it running.