Warp Weapons
In Warhammer 40,000, there are few things more dangerous to life in the galaxy than the Warp. Especially when it's weaponized. Some of the deadliest weapons in existence, Warp Weapons are devices capable of using the warp to deal tremendous damage to any target presented with.
Imperial Variants
Vortex Weapons come in two variations: grenades that can open holes in realspace and the Vortex Missile which is mounted on either a Deathstrike Missile Launcher or Reaver/Warlord Battle Titans and can hatefuck massive holes in reality in a single strike. A non-standard Vortex Weapon is Dominus Astra, an Emperor Class Battleship, which when strategically located and having its warp drive detonated is capable of destroying almost all of Hive Fleet Behemoth.
Xenos Variants
Eldar
Though Warp weapons are considered taboo in Eldar society for their unnerving effects, Eldar military forces always seem to find that they don't really give a good goddamn when it comes to defending the craftworlds, and as such, make use of more Warp weapons than anyone else in 40k.
Wraithcannon
Carried by Eldar Wraithguard and a certain variant of grav-tank, these guns are just like the Imperial Vortex weapon in that they open a portal and drag anything near it to the warp. They are powered by the Wraithguard's psychic energy of its spirit stone, the Wraithcannon opens a small temporary rift between real and warp space, tearing apart its target as it is torn between different dimensions.
However, unlike the D-Cannon the Wraithcannon is more tightly controlled and more accurate. The Wraithcannons are so fiendish that no mortal (only Wraithguard) could wield them. There's also the Heavy Wraithcannon, which has triple the range, and is the basic weapon of the Wraithknight.
D-Scythe
A variant on the Wraithcannon that is dispersed instead of focused, so it can hit more enemies at once but does less damage. There is once again a heavier variant called the Heavy D-Scythe that's mounted on the Hemlock Wraithfighter.
Considered a sinister and taboo weapon, D-Scythes can rip the soul of an enemy from their body and banish it to the Warp. When fired, a D-Scythe emits no visible blast and leaves behind no damage, save the motionless corpse of those targeted. D-Scythes are normally wielded by Wraithguards in place of their Wraithcannons.
D-Cannon
A larger version of the Wraithcannon mounted on a special artillery platform, the D-Cannon can collapse realspace in a small area and suck in all unlucky sods that stood too close to the hole. You want it.
The size of the warp hole created depends on the size of the D-Cannon, with larger weapons requiring more power and a larger chassis to mount them. The largest D-Cannons are those mounted on Eldar Cobras and Phantom Titans, and even Void Shields are useless in defending against them. These terrifying weapons are capable of swallowing multiple battle tanks in one hit, and the shockwave that results from their sudden closure can knock over other armoured vehicles lucky enough to avoid being dragged into Hell.
The AdMech tried numerous times to reverse-engineer and study the mechanics of the D-Cannon. Failures, one too many.
Orks
Shokk Attack Gun
The Shokk Attack Gun is one of the only Ork-portable weapons that force the wielder to stay still while firing. It opens a conduit through the Warp to the target and shoves the ammunition through it. Orks like to use Snotlings for this; because they travel through the Warp unprotected, they get scared out of their minds by the daemons, and when they come out, they usually rip and tear their way through anything in front of them. It's crazily unpredictable though. It can do anything from killing everything within D6" of the Big Mek, including the Mek himself or rolling double six and utterly destroying anything under the template.
In appearance it is a large, heavy piece of machinery with huge, spinning blades at the front and numerous whirring, grinding cogs and other gubbins sticking out from the casing. When used it makes a distinktive whine, shakes and rattles in an alarming fashion, oily smoke pours from inside as its Mek Operator holds on as tight as he can. It is very hard to aim with a Shokk Attack Gun, as even the strongest Orks find it hard to hold it still.
Hrud
Warp Fusil
The Hrud use a type of warp-laced plasma to create a potent sniper weapon called the Warp Fusil. The plasma shot dematerializes from real space for a moment, allowing it to bypass barriers and armour, before emerging into reality a moment later and making a gooey, charred mess of whatever was aimed at. Because Hrud infest everywhere and their warp fusils are excellent at assassinating things, they see use among many alien and imperial mercenaries.
Kinebrach
The Kinebrach used to make weapons at least some of which required interaction with the Warp or Warp entities during their creation. Some of the weapons were Cubes. The production of many, if not all of their weaponry was later considered forbidden. The Interex showed these weapons to the Imperium, despite suspecting the Imperium being tainted by Kaos (their word for Chaos). This may have been a test to see if they were actually tainted by Kaos, an attempt to impress the militaristic Imperium, or both.
Old Ones Variants
During the War in Heaven, the Old Ones discovered that the C'Tan and their Necron armies were vulnerable against psychic forces. To capitalize on this weakness, they had the brilliant idea of weaponizing the Warp, thus turning it into the twisted shit-pit it is now. Of particular note is the Shadowlight, believed to be the power source of some larger assembly. The power source alone is capable of making psykers more powerful, latent psykers become psykers, and anyone else dies a gruesome death upon unprotected exposure to it (although psykers, latent or otherwise, might also suffer said gruesome death). According to a psyker inquisitor, if Chaos got their hands on it, there would be a lot of unsanctioned psykers running around (duh), and without the Emperor's protection they would get possessed, and within 2 generations the Eye of Terror would expand so large that it would engulf the entire Imperium. It is suspected that the assembly "can alter the very fabric of reality". This suspicion proved correct, as later it opened a portal to the warp, and a daemon came out. The assembly's intended purpose is unknown, but it is suspected to be designed to somehow help the old ones in the war against the C'Tan.
Blackstone Fortress
Each of these weapons was a star fortress equipped with main guns that made the Death Star look like a fucking pea shooter. Drawing upon the warp and channeling staggering amounts of power, they fired beams of pure psychic energy that caused anything in their path to suffer critical existence failure. These things were anti-star weapons! During the Gothic war, Abaddon used three partially re-activated Fortresses to destroy the entire Tarantis system by blowing the local sun a new asshole, causing it to go supernova. Quite characteristically, Abaddon still manages to lose the war with such powerful weapons.
Somehow, the Void Dragon survived a combined blast from multiple of these things and all it did was make him a bit drowsy. WOW. Then the Big E kicked him in the balls. Which gives you a pretty good idea of how shit of an idea it is to piss off the emprah. Except the Emperor fought only a mere shard of the Void Dragon, not the fully-formed C'Tan, which is literally a god, as powerful in the material realm as the Chaos Gods are in the Warp.