How long was the flood out for? Who awakened it?
The Flood go wayyy back. I don’t think anyone knows how old they are. The Forerunners encountered them a long time ago.
Also, I don’t think The Flood ever went to sleep. I think The Gravemind was just waiting for the right time to attack.
I’m no expert though. And I haven’t read Cryptum either so maybe there are some answers in there.