"Listen, and understand. That terminator is out there. It can’t be bargained with. It can’t be reasoned with. It doesn’t feel pity, or remorse, or fear. And it absolutely will not stop, ever, until you are dead."
- – Kyle Reese from the Terminator film franchise
Necron Destroyers (now called Lokhust Destroyers following the introduction of other Destroyer subtypes) are Immortals who go buttfuck nuts during their stasis. They awaken filled with nothing but nihilistic hatred for the entire universe and everything in it. They especially hate all lifeforms (they have the special rule of "Preferred Enemy: Everything!" -- yes, the rule entry does have an exclamation point, which is awesome.), which is not unusual in itself, but they are also willing to modify their own bodies to make themselves more efficient killing machines.
Some Necrons lords have theorized that if everything else dies, the Destroyers will turn on their fellow Necrons and maybe even each other. The aforementioned self-modification tends to involve replacing one arm with a massive gauss cannon and replacing their legs with a big hoverboard. Back in Third Edition, Destroyers (and Heavy Destroyers) were jetbikes, but the Fifth Edition Codex split that aspect of their character into the new Tomb Blade unit, and the Destroyers became Jump Infantry, however in 7th edition they became jetpacks.
UPDATE: The new codex made them into jet pack infantry and removed the "Everything!" part of their preferred enemy rule (although functionally the rule remains the same).
Lokhust Heavy Destroyer
Heavy Destroyers (Changed in 9th to be the Lokhust Heavy Destroyers to differentiate from their newer brothers) used to be a Heavy Support choice for Necron armies, despite having exactly the same stats as regular (Fast Attack/ 7th Jet pack infantry) Destroyers except for a more powerful gun.
In a stunning display of common sense, Games Workshop realized that splitting them up was silly, and in the Fifth Edition Codex, all Destroyers are under one Fast Attack choice: up to three Destroyers in a squad can become Heavy Destroyers, upgrading their gauss cannons to heavy gauss cannons to have fun hunting tanks. Games Workshop then clearly went retarded again and in the 7th edition codex put heavy destroyers back into the heavy support section, but still allows one (yes ONE) model in a fast attack destroyer unit to upgrade to a heavy destroyer.
8th changed this yet again: now a destroyer in a fast attack unit can upgrade to a heavy destroyer as long as there are at least 3 destroyers in the unit.
From a practical standpoint splitting them into two units works. Heavy Ds have longer range and a weapon dedicated to tank and monstrous creature hunting, where Destroyers suffer from Necron 'short' range and excel at MEQ hunting.
9e gives them a choice of two different weapons- the horde-blasting Enmitic Exterminator and the tank-killing Gauss Destructor. Also brings all the Destroyers in Heavy support, with the choice of either adding an Heavy Destroyer in a unit of 1-6 regular Destroyers (adding, not replacing, so the unit goes up to 7) or taking only Heavy Destroyers in 1-3 units
- Heavy-chan, a Heavy Destroyer who is Lolicron's older sister (though she insists that she's NOT HEAVY, just "big-chassied")
- Skorpekh Destroyers, their heavy melee focused counterparts.
- Ophydian Destroyers, their pre-5e Wraith impersonating cousins.
- Hexmark Destroyers, what happens when you cross a Destroyer with a Deathmark.