Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150406151006/@comment-25177957-20150406193018

The Necrons are unbeatable by the Tyranids technically, and users on this wikia have discussed why before, since the Necrons and Tomb Worlds yield no biomass, Tyranids who engage the Necrons won't be able to replenish their losses, also because Necron gauss weaponry destroys matter down to the molecule (or atom), and the Tyranids won't be able to harvest dead Tyranids either.

The Forces of Chaos are pretty much safe from the space locuses, the Chaos Gods are secure within the warp (unless the Tyranids manage to breach it with the Shadow) and the Chaos Gods have nearly infinite daemons and mortal servants. Daemon "biomass" cannot be harvested by the Tyranids since daemon physical structures vanish once a daemon dies. Other than the daemons, the traitor legions and traitor Primarchs would give the Tyranids plenty of trouble, especially the Iron Warriors and Black Legion.

Now the Orks are what could stop the Tyranids dead in their tracks, but not on their own volition, and the results of this would be horrid for the galaxy, I will quote a theory from 40k fanfiction here, but it's certainly valid I believe. Let's say that the Octarius war reaches it's peak and the Tyranids begin to harvest Ork biomass and Ork DNA, this means that Hive Fleet Leviathan will become half-Ork and half-Tyranid essentially, having the species bonuses of both, yet the weaknesses of neither. If the half Ork and half Tyranids are able to form their own Hive Mind (Leviathan right now is far away from any other Hive Fleets and deep within Imperial borders), the half-Ork and half- Tyranid hybrids would essentially be invincible, able to grow indefinately no matter the circumstances. This new threat could drive the pure Tyranids out the galaxy, and bring the Orks into extinction (the waaagh would call them all to fight the Tyranorks).

The Imperium, Eldar, and Tau are all pretty much doomed in the event the Tyranids arrive in force, or become Tyranorks, the Imperium is crumbling as it is, and the Eldar will wither away to nothing soon. The Tau Empire holds strong at the moment but will be wiped out by the Tyranids or Necrons at some point soon. Most likely the Tyranids.