Stolen from Remixes and Revelations and updated for 5e. One of my players found this, but, due to a magical mishap, the trident was turend to pure-lead and lost its magical nature. Here it is in its original 5e converted form.
The Prismatic Trident
This is a trident with a handle made of solid iron and covered in white gold, the tip being made of flawless crystal. The trident’s crystal head shines bright in the light and throws rainbows through the air, so any attempt to use it while sneaking near a light source should automatically fail unless the enemies are blind or will ignore bright lights and colours for some reason.
The trident functions perfectly well as a trident and is magic, so it can damage non-physical creatures such as Ghosts and the like. However, the trident has an additional ability. It functions as a staff for arcane spellcasters and can store a spell, but it will come pre-loaded with the spell prismatic spray. If you are a spellcaster you can cast the spell loaded into the staff.
However, if the Prismatic Trident is empty and the person holding it casts a spell that requires an attack roll if they miss, the Prismatic Trident gives them the ability to reroll that attack roll, though they must use the second result.
There is a catch though. The Prismatic Trident is actually sapient and highly intelligent. It calls itself Emperor Locus or The Refracted King and demands your respect. It prefers to be addressed as “Your Majesty”, but any sufficiently respectful title will probably be enough to sway it. It has better things to do than nitpick you. Locus does not tolerate disrespect and if it is not treated well, protected and addressed politely and courteously, it will deny any disrespectful person who uses it the ability to reroll his spell attack rolls. In particularly egregious cases it might reflect the attack back at the caster.
The Prismatic Trident will also make requests of its wielder from time to time. Each day, it has a 2-in-6 chance of making a request, with 1d4 weeks, or however much time the Referee thinks is appropriate, till the next request is made. These requests will begin small and seemingly arbitrary, but will gradually increase in magnitude. First, the Prismatic Trident will ask you to acquire a rare book and give it to some seemingly unimportant bureaucrat, then before you know it it will be asking you to hijack ships carrying large shipments of metal ore and disrupt local economies. These requests will be staggered, with each one increasing in scope and wickedness, though the Prismatic Trident doesn’t seem actively malicious. It doesn’t seem to care about mortals, or even individuals, at all. To it, you are tools to achieve an end.
The Prismatic Trident also has two secret abilities that it will not tell you about. The first is that it can cast prismatic wall once per day, either on just itself or on itself and the wielder. If you anger it, the Prismatic Trident will cast this spell on itself to cause anyone who tries to touch it will have a very, very bad time.
The second is that the Prismatic Trident can cast teleport once per day, either on itself, itself and its wielder or itself, its wielder and up to [sum] other creatures who are touching the wielder or someone who is touching the wielder, such as if everyone is standing in a circle and holding hands. The spellcasting dice used to expand this spell must come from the current wielder.
If you are in serious danger and the Prismatic Trident feels you will be useful to it, it will use these abilities to save you. If it does not, it will not reveal these powers and simply let you die. If it fears for its own safety though, or that it might end up in some terrible place, it will use these abilities to escape and leave you to die. It has no real loyalty.
The Prismatic Trident is very tough and is immune to most elemental forms of damage, necrotic, radiant, cold, acid. It also cannot be damaged by non-magical weapons or tools. The only way to destroy it would be to hit it with a magic weapon that was just as or even more powerful, exposes it to some kind of effect that destroys magic utterly, or melt it into slag. It can resist most fires without taking any damage, but a sufficiently hot blaze, such as a furnace used to make high-quality steel or dragonfire (assuming you can find a dragon that breathes fire and not some other nonsense) could do it. Of course, this is all secret knowledge. The Prismatic Javelin will never disclose its weaknesses.
Here is another piece of information the Prismatic Trident will never disclose. It is actually the phylactery of an ancient Lich named Locus, the Refracted King. The weapon itself is not intelligent, being merely connected to his soul and you were actually conversing with an ancient Undead spellcaster who traded everything, including his “humanity” for power. Locus will do anything to protect the Prismatic Javelin as if it is destroyed, so is he. This is why he imbued it with so much power, so it couldn’t easily be destroyed. He could recall it at any time, but he feels that it is better served to be as far away from him as possible.