The Skull Cannon is a Daemon Engine used by the Daemon Legions of Khorne. The means of its creation are unknown, but it seems that the chariot itself is a daemon, whose bloodlust equals that of its pilots. It fires skulls at his enemies, which are so full of Rage they catch fire. Although it is an artillery piece, it is still a daemon of Khorne, so is no slouch in melee: its vicious spikes rend flesh and break bone, and it crushes any enemy infantry foolish enough to try to try to assault it, while its pilots claim skulls for Khorne with their Hellblades. In fact, it's in melee where the Cannon obtains its ammunition: it munchs on its fallen victims with his half metal half flesh mouth, then takes their skulls and shoots them. So, it's a war machine that wants to be close and personal: you can say that Khorne doesn't truly understand the concept of cannon...
Although Khorne is the god of RAGE, he is also the god of battle, and his daemons are among the most disciplined of the Legions of the Warp (outside of melee at least). When his legions finally break formation and charge into melee to RIP AND TEAR, Skull Cannons will provide covering fire and claim skulls.
On the tabletop
The Skull Cannon is very mobile for a war machine, because it can move and still shoot. And because these are Flaming Attacks (the blazing skull things, remember), it's particularly good against big regenerating Monsters or Treemen. Very good, particularly for Chaos, that has very little in the way of shooting, and Khorne, that has basically none.
Age of Sigmar
Here is where this contraption is truly a thing of Khorne: the cannon WANTS to go to melee, because if it kills something with his chariot mouth (some like a 40-50% chance of it happening) it can make an extra shot right there and then. And you can even shoot a different unit that the one you're fighting, madness, I tell you! And with 7 wounds and 4+ save the Cannon can take some abuse, too, so it isn't shy of combat. Unless you need it to kill some inaccesible crucial unit, guide it to the biggest blob you can see, because it also gets +1 to hit with its shoot against units with 10+ minis. Dangerous and versatile, it won't let you down if you use your head with it (wink wink).
Rules-wise, the Skull Cannon is an open-topped Chariot with a transport capacity of 0, BS 5, 3 HP and AV 12/12/10. It has the usual Daemon of Khorne and Deep Strike rules, and Gorefeast and a Skull Cannon. All of this for the cost of a Maulerfiend.
The Gorefeast rule means that when the chariot inflicts 1 or more unsaved wounds with its Hammer of Wrath hits, roll a d6. On a 4+, it regains 1 Hull Point. The Skull Cannon itself has 36 inch range, S8, AP5, Heavy 1, Dreadskulls, Ignores Cover and is a Large Blast. The Dreadskulls rule is fantastic, and essential for basically any infantryspam Khorne armies. It reads: "Place a marker on each unit that takes one or more hits from this weapon. Any friendly Daemon unit that charges a unit so marked does not suffer the Initiative penalty when charging through difficult terrain." Remember how one of the main flaws of Khornate daemons was lack of assault grenades on a dedicated assault unit? Yeah, that problem's solved.