Helltides spawn every 2 hours 15 minutes, and last for 1 hour.
Living Steel Chests (280 embers) are needed to spawn uber bosses. The pink chests are the locations.
During Helltides, there are mysterious chests (green chests below) that cost 250 embers. They refresh on the hour.
Dedicated page @ /events/helltides
Helltide spawned @ , ends @ , next one @

Boss : , spawn @
Following Boss : , spawn @ .
NOTE - This image is only valid for 30 minutes before the boss is about to spawn!

Legion spawn @
Legions are every 25 minutes

Whispers