One of the PCs in my campaign’s primary weapon is the Heavy Crossbow and they’re currently 5th level. I created The Siege Crossbow to give to the group during their 6th level adventure. It gives them a magical heavy crossbow which Wizards didn’t include in the normal list of magic items and it fires a ballista bolt instead of a crossbow up to 5 times per day, but it takes 2 rounds to fire the ballista bolt. Heavy crossbows do 1d10 damage and can be fired once per round, ballistas do 3d10 damage and can be fired once every 3 rounds, so even without the +1 to hit and damage the character who is using this item will be able to do more damage than a ballista for 10 rounds a day. 

Any feedback please? Is it over powered, under powered, or just about right for a 7th level PC? And will it age well so that the PC will still like it at higher levels? The campaign is Ghosts of Saltmarsh so a magic item that can overcome the damage resistance that ships have will be useful at times. 

What class is the character who's getting this weapon? If they're a class that gets multi-attack, if they take the Crossbow Expert feat they'll have no reason to use the siege feature, other than the range increase. Honestly... I would reduce it to 3 charges a day and let the player do it on a single round. Maybe add a Critical Failure feature on it of some kind... like if you roll a 1 on a Siege attack you're unable to use the Siege feature again until the next dawn.

What he said. Or you could make it take “two attacks” (because of needing to aim or whatever) so that a character with Extra Attack and Crossbow Expert could do it in one turn, (a fighter might even be able to do it 2ce at high enough level) but anyone without Extra Attack would need two turns. But that makes it more complicated so you are probably just better off doing it Transmorpher’s way.

Also, is it supposed to do 3d10 and then add the +1 because it’s magic? Because the way you have it it reads like it would do 3d10+1, and then an additional +1 to attack and damage rolls for being magic. 

The character is a Ranger and they will have multi attack. The benefit of the siege feature is for attacking ships because ships have massive amounts of damage resistance. So do buildings. It will also be useful before combat starts, but in normal combat it won’t be used much. I’m trying for a +1 weapon that has an extra feature that isn’t overpowered and unbalancing to make it special. 

3d10 three times a day is not OP. A Rogue can do way more damage with sneak attack over the course of 1 encounter. Having it take two whole rounds is a lot. A Ranger can do more damage with Hunter’s Mark and a longbow without having to take the Crossbow Expert feat because of Extra Attack. A level 6 Warlock can do 2d10/turn with Eldritch Blast, a level 6 Wizard could do 2d10/turn with Fire Bolt and those are both Cantrips. You do you, but I don’t think that’s OP in combat.

Okay if the intent is to be mostly used out of traditional combat against ships and buildings I actually think you should have it deal additional damage against objects and constructs. Like, your original idea works but it should be even better at the specific function it's designed for. Having it take two actions to fire should be treated almost more like firing a cannon... Something that's very difficult to do in melee combat but does absolutely devastating damage.

Yeah, like maybe add Fire Bolt on top of it so it would do 3d10 piercing damage and between 1-4d10 fire damage depending on the level of the wielder. That way it can set ships on fire too.

Or just give it the Siege mechanic:

“Siege Engine. This crossbow deals double damage to objects and structures. ”

 In fact, just making it a regular +1 Heavy Crossbow with the Siege mechanic would probably do everything you want but be way simpler and save you a lot of work. Then you could just give it 3 charges and let it cast Fire Bolt on top of the piercing for a charge and it’s a ship killer all right.

I took the damage from the ballista in the DMG. A ballista does 3d10 damage and it’s rate of fire is once every 3 rounds. So this is effectively turning a heavy crossbow into a ballista that fires 50% faster than a non-magical ballista fires and also making it a +1 magical weapon in normal use. 

Ah, but the ballista in the DMG is designed more as an environmental hazard rather than a usable object that players would use regularly. It makes sense as a stationary object that contributes to a specific encounter, either as a hazard that the enemy uses infrequently, or something the party members can use at key moments in a large conflict. I think the damage is a good base to build around, but the time tied to ballista doesn't scale well for a magic weapon.

Yeah. The reason a ballista took 3 turns to load is because it took 2-3 people to use and the ammunition was as large/larger than a javelin. A Heavy Crossbow deals 1d10 damage. Allowing it to do 3d10 up to 3 times per day at its normal rate of fire is fine, but with Crossbow Expert that might be a bit much, especially if there’s a Fighter in the group (Fighters get 2 Extra Attacks /action at 11th level after all), but if it worked something along the lines of:+1 to attack and damage rolls, deals double damage to vehicles and structures, and 3 (maybe 4 charges?) recharge 1d3 (or 1d4?) on a long rest and “As a bonus action you can spend 1 of this weapons charges to add an additional 2d10 fire damage to the next attack made with this weapon.” then I think it would be okay for a 5th-10th level party. That’s around the time they should be getting cool stuff like that IMO. If you want to nerf it a little then link the Siege ability to the charges too.

Keep this in mind. For any character with the Extra Attack class feature the Longbow is typically a better choice than the Heavy Crossbow because of the Loading property. Most players only use the crossbow for characters that will never get Extra Attack because only being able to fire 1ce/turn is not a disadvantage. Or they use them on a Fighter because they get up to 4 Attacks/Action so they get the most use out of the Crossbow Expert feat.

A ranger would have to either give up their second shot to use the H Crossbow, or take an entire feat to maintain the same rate of fire they already get from a longbow. Since the longbow is 1d8 damage instead of the crossbow’s 1d10 all they sacrifice is an average of 1 damage/attack without having to take the feat. Are you sure a crossbow is the best weapon to give a Ranger? If there is a fighter in the group, would you be disappointed if the party ends up giving this crossbow to them instead of the Ranger? Just things to think about.

Thank you for the feedback. 

 I think the weapon as is works fine.  3 to 5 charges per day doesn't make it overpowered.  A character with multi attack and not having the crossbow feat may not be the best person to use it.

I think functionally, instead of flipping up a sight, I would describe the ballista function of the crossbow thusly: The crossbow has a lever or pump action on the bottom.  As an action a character can lever or pump the action which opens an interdimensional pocket that has a ballista in it.  The bolt of said ballista then protrudes out of the action of the crossbow and links the triggers.  Upon release of the bolt the interdepartmental pocket closes.  Reopening it requires and action again.