Ballista

Inanimate. The ballista is an inanimate object and as such has the following properties:

• The ballista cannot recover from the prone condition without the help of other creatures. A mounted ballista (like one upon a battlement) may not be knocked prone.
• The ballista weighs 400 pounds and must be pulled by another creature(s) if it is not mounted, and if it is mounted it cannot be moved until unfastened from its position. It takes 10 minutes to mount or unfasten the ballista. It may be used either way.
• The ballista must have another creature use their action in order to perform one of its actions; it may not act on its own.

Siege. The ballista deals double damage to objects and structures.

Reload. A creature within reach of the ballista may use their action to load a ballista bolt or load and ignite an incendiary bolt that is also within reach of the creature to be fired from the ballista. 

Aim. A creature within reach of the ballista may use their action to change the space the ballista will fire at. The ballista will fire towards this space on all of its attacking actions unless it is aimed at a different space within range.

Bolt. Ranged Weapon Attack: +6 to hit, range 120/480 ft., one target. Hit: 22 (4d10) piercing damage. This action may only be used if the ballista has been reloaded with a ballista bolt since it's last fire action.

Incendiary Bolt. Ranged Weapon Attack: +5 to hit, range 120/480 ft., one target. Hit: 22 (4d10) piercing damage. Upon hit, all creatures within a 10 foot radius including the target must make a DC 18 Dexterity saving throw or take 11 (2d10) fire damage and 11 (2d10) force damage. This action may only be used if the ballista has been reloaded with an incendiary bolt since it's last fire action. If the bolt is wetted or submerged before this attack is completed it must be reignited or the damage is treated as though it were a normal ballista bolt.