Game Breaking Magic Item (Rift Blade)
Back in the late 1990s, when Traykon was just a demi-plane used by the character Morval for magical experiments, one of the Dungeon Master's allowed my character to create an item that proved to be overpowering and campaign breaking in many ways. In 2nd Edition Dungeons and Dragons, the Plane Shift spell was somewhat unwieldy but incredibly powerful. A few campaigns ended early when my priest Morval was able to shift enemies out of his plane into various hostile dimensions. With some well laid plans and competent allies, enemies would often find themselves in a situation of being shifted into one of the energy planes or into a wholly unhealthy level of the Abyss. Since my character was also a master crafter of magical weapons and equipment, I was allowed to craft magical items often unimagined by even the most prolific fantasy authors.
Enter the Rift Blade
Despite being a priest, Morval was a front line fighter due in large part to his devotion to a War God. While other players equipped their characters with staggering levels of magical armaments, Morval instead chose to focus on a few items to maximize his combat prowess. One of his favorite tactics that developed during high level campaigns was for him to prepare the Plane Shift spell before combat and shift opponents into oblivion. This tactic became so common that I hit upon the idea of making a sword bound to the theory of Chaos in that it would have the ability to create a Planar Rift capable of pulling those struck into another world similar to the Plane Shift spell.
For those unfamiliar with the Planar Rift spell, it worked exactly like Plane Shift except it formed a tear in the fabric of reality to another plane of existence. The rift would only last for a moment, but during that time anything in contact with the rift would have to save vs death magic or be sucked through. There was no magic resistance save nor was there an easy way back once you were sent through the rift. I am not sure where this spell originated but it was a "combat version" of the Plane Shift spell. Since it led to a random plane, using it for planar travel was impracticable.
Morval crafted a sword with this spell bound into the blade. Using the same criteria for a Vorpal Sword, this blade would open a rift only upon a critical hit. Since the blade itself open the rift, often the rift was opened within the wounds of an enemy. There was no chance to dodge it and a very low chance to make your save due to associated penalties. The major two drawbacks of the weapon were related to it's function.
The main problem with the weapon was that it was unpredictable as to when the Plane Rift would open. During combat, a rift could be opened at the very start of battle and cut the encounter short, or it could open at the end when the enemy was already about to fall over. There was also a good chance that no rift would open at all, meaning the sword simply acted as a normal magical weapon with no other bonuses. The other problem, and some would say the biggest problem, was that when the Planar Rift removed an enemy, all the equipment that enemy possessed was lost as well. It was as if the enemy was the target of a disintegrate spell, there simply was no loot. This last reason was the one that caused party members to really dislike my using the Rift Blade. Since my character had no concern for loot though, it was something that didn't matter to me.
The most notable use of this weapon that broke a campaign was when our party was trapped by a Great Wyrm Deep Dragon who was also the Chosen of an enemy deity. We had to fight through his lair of enemies and traps. At one point, we encountered an enemy party (with the Deep Dragon polymorphed) who were supposed to weaken our party down. The DM intended this encounter to drain our party of resources since we had no ability to rest or resupply before the big encounter. As our party entered the room and engaged the enemy, Morval went straight for the Drow Cleric (Deep Dragon shifted) and with his first strike a Planar Rift opened and the shapeshifted main enemy for the campaign was sucked away into what turned out to be the Positive Material Plane. He was unable to return or even survive. This one encounter, which was just a minor skirmish, ended up terminating the campaign early due to the utter power of this weapon. I noted that a Vorpal Sword could have done the same, but the DM and other players were not amused.
This weapon taught me and my friends valuable lessons about the power of single items to sway campaigns. The very unpredictability of the weapon made it such a major obstacle for DMs to plan against. Morval continued to use the weapon sparingly, but by then he had started his ascension so was concerned with collecting the relics that would give him full godhood. With the advent of 3rd Edition, the Rift Blade was transferred over to new system and given different triggers and abilities. While still a formidable weapon (even in 5th Edition) the chaotic unpredictability is now gone making the weapon easier to plan for. There is also a major issue with using the current Incarnation of the Rift Blade that it can suck in the wielder and others. These additions to the original were made so the weapon could make future appearaces without destroying campaigns. Few people are willing to trigger a weapon that could in turn destroy themselves. I consider this weapon the first Tactical Nuclear Weapon of Magic in fantasy gaming.