Warp Weapons
Big FUUUUUUUUUUUU Cannons that would even make Ollanius Pius shit himself.
Old Ones Variants
Blackstone Fortress
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.
Equipped with main guns that made the Death-Star look like a fucking pea shooter. These things were anti-star weapons!
During the Gothic war Abaddon the Failure used three partially re-activated Fortress' to destroy the entire Tarantis system by blowing the local sun a new ass-hole causing it to go super-nova. (Even with that kind of power he still lost and had to retreat. Total failure.)
Imperial Variants
Vortex Weapons These come in missile or grenade form. The missile is a FUCKHUGE warhead designed to be mounted on Deathstrike Missile Launcher or Reaver Battle Titan while the grenade does the same thing packaged in a tiny little throwable case. They open a large blast template that sucks everything under it into the warp.
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 somewhat nerfed form of the Wraithcannon, the beam teleports a small chunk out of the target.
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 and uses daemons to scare the snotling ammo out of their minds. 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 like killing everything within D6" of the Big Mek, including the Mek himself or rolling double six and utterly destroying anything under the template.