Raids are boss kill based events. Appear in open-world map so it makes them pretty dangerous specially when you are in wrong time, wrong place. At the end of raid a boss appears.
Raids appear at appointed time and don't appear again until you defeat the previous one.
Sound Shinobi Raid: 06:00 10:00 14:00 18:00 20:00
Shinobi War Raid: 22:00 23:00
Akatsuki Attack Raid: 03:00 11:00 23:00
Akatsuki Attack Raid: 03:15 11:15 23:15
higher chance to drop items than its counterpart as boss; upgrade stones; unknown jutsu books