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
Wraithcannon
Carried by 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.
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.
Shokk Attack Gun
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.
Kinebrach
They used to make weapons which 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 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 the 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 alot of unsanctioned psyker's running around (duh), without the emperor's protection they would get possessed, and within 2 generations the eye of terror would expand so large 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
One 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 ass-hole causing it to go supernova.
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 how shit of an idea it is to piss off the emprah. Except Emperor fought only mere shard of Void Dragon, not the fully-formed C'Tan, which is literally a god, as powerful in material realm, as Chaos Gods are powerful in the Warp. The fluff about the Emperor beating & imprisoning the Void Dragon was from before the n00bcron recton. Even if it's now outdated, the fluff still says that he's capable of beating an intact C'Tan.