The Coiled Viper (Legendary Magical Item)

I made this item for my Tomb of Annihilation game. I wanted an item that would get better over time, that had versatility, but also packed a punch. Thematically, I knew I wanted to connect it to Dendar The Night Serpent, but that flavour is utterly interchangeable. And then lastly and perhaps unimportantly, I was intrigued in making it not require ammunition. Ammo tracking is most often handwaved, so this isn't really a feature — but it did inform my design.

The basics
The Coiled Viper is a magical +2 light crossbow. It can only be loaded with its own custom bolts and has no drawstring. 

The crossbow is then perhaps a ceremonial object, but the truth lies in its ammunition. A thin wide box holds the bolts for this weapon: each is an exquisitely carved stone snake; perhaps a little square in the design (for better fitting into the firing groove), but each is strikingly different to one another in head shape, scale pattern and the muted colour that each possess.

These stone bolts are shaped to appear as straightened snakes with closed mouths, but when they are loaded into the depressed firing groove of the crossbow, their mouths hinge open revealing their throat and fangs. The stone bolt in fact never leaves the the crossbow when the firing mechanism is released, instead the magical bowstring fires a magical projectile made of force from the loaded snakebolt, through the carved head of Dendar at the head of the crossbow. The stone snake bolt remains in the firing chamber, ready to be used again, once loaded.

In this way, it may be useful to think of the snake bolts as "magical cartridges" instead of an actual projectile. What is interesting is that each snake bolt has its own magical effect, but you need to unlock these effects with a critical hit. You get your first snake bolt: "Mamba" unlocked for free.

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Coiled Viper Description
Attunement required. Simple Ranged.
Legendary +2 Light Crossbow. 10lbs.
Properties: Loading, Two-handed, Focus (Cleric of Dendar)

At first glance this appears to be an elaborately and beautifully carved light crossbow of a darkly stained metal. Upon closer examination it is clear that this is a blessed weapon of the Yuan-Ti. Every surface is covered in tightly woven engravings of snake-people hybrids venerating Dendar, The Night Serpent – a giant snake which is carved along the length of the weapon, culminating in a sculpted restful snake head at the front of it. Where there isn't carving, there is an intricate snakescale pattern. 

It has some other unusual characteristics:

• There is no string to fire a bolt.
• The groove for the bolt is unusually deep and too wide and there is no bolt shuttle (the block that pulls the bolt forward under tremendous tension).
• The front end of the device is a beautifully sculpted close-mouthed snake's head, and its contours lie in the direct path a quarrel would be fired.

It is clear that this crossbow is unconventional or just broken. No string, no firing shuttle and no clear path for the bolt to fly.

To use this weapon in Foundry, use the ammunition not the base weapon. (In Foundry, one attacks with the snake bolt instead of the crossbow.)

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The ammunition
Depending on which ammunition is loaded, the effect of what happens when the snakebolt of force strikes its target is significantly different. There are 6 such bolts, each with their own effect. Mastery of how to use them all requires time, practice and luck. As the timing of the trigger release needs to coincide with one's own breath and the soft breathing of the snake body within the weapon, which can only be detected when a bolt is loaded and the wielder has pulled the crossbow tight into their body.

There are 8 known snake bolts for The Coiled Viper, but I have only detailed 6 of them. Each of them have a snake species name: Mamba, Cobra, Anaconda, Titanboa, Boomslang, and Mocassin. This leaves me design space down the line after I have seen the item in use in the campaign to add the last two when and if appropriate. I also didn't want to overwhelm the player with too much choice as to what to unlock and to provide some mystery for the player as to what those missing two could be. If he said, "Boy I sure wish I had a bolt that did this..." could also be a good cue to make something cool and interesting.

Firing it without attuning to it
I said this just doesn't work and that the wielder can only fire the unlearned snakebolt. You could certainly allow it with disadvantage. Whatever your decision, I'd definitely not allow any bolts to be learned without attunement. If they unattune to the weapon later after learning a few snakebolts, I'd allow them to remember that which they have unlocked. Whatever a wielder unlocks, they only unlock it for themselves.

If you spend an hour attuning, you understand what each of the 6 bolts is capable of and have unlocked the ability to use the first bolt Mamba immediately.

Unlocking & learning a snake bolt
"Mamba" is always learned and is available if attuned. Thereafter the player tells you ahead of the time what snakebolt they've loaded into the The Coiled Viper that they wish to attempt to properly learn (unlock); eg: "I've loaded the Anaconda snakebolt into my Viper." While they are learning how to properly use this snakebolt, they will use the stats for the "unlearned snakebolt" — which are the worst stats of any of the bolts. At any time they roll a natural 20 on a to hit with the Coiled Viper, they instantly learn the loaded snakebolt. If the snakebolt is already learned, then no additional learning occurs (you cannot transfer the crit to an unlearned and unloaded snakebolt: "First load, then learn.")

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All snakebolts
• to hit: +2 (inherited from the crossbow)

0. Unlearned snakebolt (to be used when the wielder has loaded a bolt they have yet to learn. In the beginning, these are all snakebolts but Mamba (if attuned)).

• damage: 1d6 + 2 + dex modifier force damage

1. Mamba (Learned as soon as the wielder is attuned)
• damage: 1d12 +2 + dex modifier force damage

2. Cobra
• damage: 1d10 +2 + dex modifier force damage
• crits on 19-20
• fumble: jams on a 1. (Requires an action to clear the jam. This action can also be used to load another ammunition type.)

3. Anaconda
• damage: no damage
• perk : Fires an invisible rope of force that adheres to a solid surface much like a grappling hook, except the hook doesn't have to hook onto anything - the force bolt securely fastens to the solid object on the ethereal plane.

A ranged attacked roll is made to determine if the attempt to secure the line successful. The DC is based on range:

Under 40' = Succeeds automatically
41-80' = 15DC
81-160' = 20DC

Once fired, a COMMAND WORD is spoken and the hand of the firer is bonded to the weapon, as the weapon climbs up the invisible rope at 60' per second. Another COMMAND WORD releases the bearers grip from the weapon, and the attached line of force from the wall. Like a quick release mechanised pulley system. 

4. Titanboa
• damage: 1d8 + 2 + str modifier force damage
• perk: Hit the target with a ranged attack. Target must make a STR save vs DC16 or be pushed back 20' and be knocked prone.
• fumble: On a natural 1 the concussive force erupts at the point of the weapon, causing the wielder to make a STR save vs DC16 or be blown backwards 10' and fall prone.

5. Boomslang
• damage: 1d8 + 2 + str modifier force damage
• perk: Fires a force spike that impales the target for regular force damage and at the start of your next turn explodes in a 5' radius for 5d6 fire damage. All within 5' of the initial target must make a DEX save vs DC14 for no damage. The target automatically fails their save.

If deliberately fired at a physical structure/object, it embeds itself and at the start of your next turn, implodes for 10d6 fire damage. The implosion occurs within the structure, so even someone standing right next to the structure would not be affected by any blast radius. It is consider to have the siege weapon property (although its double damage against objects has already been calculated)

6. Mocassin
• damage: 1d8 + 2 + dex modifier force damage
• perk: Poison damage (serpent venom): Target must make a CON save vs DC11 or take 3d6 poison damage.
• fumble: Jams on a 1. (Requires an action to clear the jam. This action can also be used to load another ammunition type.)