- last post: 01.01.0001 12:00 AM PDT
All of the transmissions in the Halo 3 teaser were from the Gravemind, but he used Cortana to send them.
When the Gravemind says "This, is the way the world ends" he means all sentient life within the Milky Way Galaxy (blast radius of the installations) will end.
"I know you, your past, your future." means that he knows the Forerunners and how the survivors became the Human race, and he knows the outcome of the Covenant - Human war.