Also known as the Fuck-you-on Arrow, the Silver Assrape, Mister Pint-Sized Powerhouse, The Necron Proverbial Middle-Finger, The One-Punch Arrow, Strong Enough to Make an Exitus Rifle Look Limp, "I-Cannot-Believe-This-Puny-Thing-Has-More-Firepower-Than-Some-Titan-Weaponry!", etc, etc, yadda, yadda, yadda, you get the idea.
A Tachyon Arrow is an intricate Necron wrist-mounted directed energy weapon worn by Necron royalty like Necron Overlords, Necron Lords and Necron Destroyer Lords. When activated, it transmutes a sliver of inert metal into an unstoppable thunderbolt capable of piercing the heart of a mountain.
Such a weapon has near infinite range, and is able to penetrate almost any form of armor, including that of Titans. The only disadvantage to this astounding item is that its strange ammo supply is exhausted with a single shot, and is far too complex to be reloaded in the midst of combat.
If you have a need of a small, wrist mounted sidearm that can make Emperor Battle Titans bend over in shame, there are always the Necrons to have your back.
It is canon that a Necron Overlord has fired a Tachyon Arrow at a Warlord Titan, striking its reactors and causing it to detonate.
On the table, every Tachyon Arrow is S12 AP-5 D6. Sounds good, especially since every unit holding one has a 2+ BS, right?
There are three opportunities (four if you count your opponent passing his invuln) for you to roll a 1! This has happened so often that Tachyon Arrows have joined Hunter-killer missiles, seeker missiles, and other one-shot weapons, in that they’re powerful, but the change of that power being unleashed upon the target is relatively low.