I think it's a joint effort from TEC, Advent and Vasari from the future.
The future TAV coalition discovered, after billions of years, close to the death of our universe, that it would be possible to open a rift and jump into another, younger universe. But they were already beyond the universal expansion curve where resources were already to far away an rarefied inside or universe, to the point that gathering energy spent more than it collected, beyond any hope.
They discovered, though, that they could use the same principles - which are connected to a much more advanced technology, based on the principles of phase jumping - they could use all their remaining energy reserves to send a small group back in time.
The objective would be to form a TAV alliance long before the point of no return in the universe expansion curve, so that resource can be gathered for the interverse jump.
Trying to contact people from the past to prepare their coming was disastrous though, since they fed them too much information, and that caused severe mental instability on the listeners. Messages were often misunderstood as threats of destruction from an 'alien' species, which are only alien from that time, being the descendants of the listeners.
Eventually the TAV coalition reluctantly decided to do a violent incursion, knowing very well that their primitive ancestors were engulfed into an economy of war, and were too belligerant-minded to listen to reason.
With broken hearts, they jumped into the past to wage war on their ancestors, with their hearts sunken into a deep sorrow for each of their beloved ones they had to kill, for their own survival.
That is their sin, that is their sacrifice, even though they are ones sacrificing themselves, for they will not survive this universe's end, but they meet their fate gracefully in the hope that those who came before them learn from their mistakes, and run for their survival while there's time.
In truth, they are fighting as a mighty force, which no single race can defeat, so that their ancestors can form and fight as the TAV alliance, against this mighty TAV coalition (from the future), which is actually fighting a war they wish to lose with all their hearts, but will only accomplish what they need to if they fight with all the fiber of their will.
[time-travel plots are mind-twisting eh?]
