Finaly finished it ^^; Took me quite a while actualy - second epic gifty for  lynneth and  darknessrisen

How they ended up like this, he did not know. Nonetheless, it somehow had happened. He knew that Space Marines were fierce enemies, wielding backwards yet powerful weapons. Should the Imperium ever be in possession of the plans for more advanced weapons, they would surely be able to quickly dominate the galaxy.
Meanwhile, the ancient machines he and his comrades were fighting, the Necrons, had incredibly powerful Gauss weaponry, capable of punching through almost any known armor, despite being older than Mankind. At the same time, the libing metal from which they were made, made them night impossible to kill. Some of the more powerful Necron lords were even capable of resurrecting fallen Necron Warriors, from what he knew.
And now, he, Commander Farsight, renegade Tau warrior, fought together with some of the most powerful and fiercest of all Space Marines; the Grey Knights. He knew little about them, but easily recognised the powers these men brought into battle.

Together with Space Marines against Necrons.
Usually, that's be nigh impossible, but a sudden attack about 10 seconds before the two parties began slaughtering each other seems to indeed be capable of forcing Grey Knights and Tau renegades to be fighting together. Of course, there were "accidents".
Well, how does the saying go? Living metal happens.