Rifts are boss kill based events. You have to enter to special area, survive and at the end kill a boss.
Special enters appear at appointed time and are opened only for 15 minutes.
Lava Rift(50+ level): 14:00 22:00 23:59
Ice Rift(125+ level): 06:00 12:00 15:00
Sand Rift(200+ level): 08:00 17:00
Grass Rift(300+ level): 21:00
Earth Rift(400+ level): 22:00
Special enter looks like
These enters are located:
Black pebbles; exceptional quality items(best quality); upgrade stones; unknown jutsu books; experience