We don't know where they came from, we don't know what they are, all we have is a name. Titans. They now roam the earth leaving paths of destruction from their sheer size, sizing up to our cities reaching far into the clouds.
We soon figured that the human race wouldnt go extinct from our own greed, no war, no disaster, only time. They stand as sentinels waiting for us to go, it's not our world anymore.
When the first appeared every religion marked it the beginning of the end and in a way they weren't wrong, a sign from god was what they called it, rupture even.