For those who didn't see it:
The man in the sneak peak narrated the preview, with explosions in the building that he was in. He sat up, injected himself with something along the lines of an adrenaline shot, and used the scope to see some innocents running away. He glances left, and shoots into some dust, killing a creature unseen.
He mentions how Sheppard better get things together, and you see reapers walking(?) on the earth's surface, destroying everything. Cut to Sheppard, watching the destruction from space.
then,
MASS EFFECT 3.
Please help if I left out some details.