Cynical Cat said:
Tyranids are very dependent on the warp (their hive mind and everything). The Necron Great Ward plan and various warp suppressing devices like the Pariahs and the Cadian Pylons are definitely going to go over poorly with the 'Nids. Given the huge size and advanced technology involved in a Dyson Sphere, there could be all kinds of things the 'Nids don't like about the sphere and that's before we bring in the C'tan.
As has been mentioned, space is three dimensional and the 'Nids have been invading the galaxy from different directions for thousands of years. Its perfectly consistent with the established background that a fragment of a hive fleet ended up in the vicinity of Calixus.
I question that. It's possible, but highly unlikely given more recent canon on 'nids, that a splinter fleet might end up in the vicinity of Calixis.
Yes, there are some (now highly contradictory) vague incidents of 'nid activity in the northern galactic region, however they do seem to be very few and originating in older (arguably superceded) canon.
I think any kind of major 'nid activity, splinter fleet and up, in the general Calixis region, in the 815.M41 era, does indeed clash with what's written about 'nids in more recent and broadly accepted canon.
Just my personal view. On the other hand, a GM is god at his own table, so go for your life. I'm not sure 'nids make great rpg villains anyway mind.
My 40k photomanips -
The "Nids are attacking the galaxy from another galaxy and from several different directions. Compared to the vastness of intergalactic space, skipping across the galactic disk is nothing. The Tyranids dispatch other attack forces besides hive fleets, such as genestealer infiltrators and lictor scouts. If Deathworlds such as Fenris are truly influenced by the presence of Tyranid life forms, as some believe, the invasion is well over ten thousand years old. That's more than enough time to have elements in all corners of the galaxy if the GM so desires.