This is a “warhead” for an ordinary ballista bolt made up of three boxes of differing size. The first box (the casing) is made of balsa wood or some other “soft” or weak wood. It is made to contain two other boxes, one smaller than the other. These other boxes are made of metal or some “hard” wood, resistant to sharp impacts. The warhead must be prepared before it is fired. The two smaller boxes must have the Level 5 priest spell Extradimensional Pocket cast on them (ToM). They are then arranged in the casing so that the smaller box faces the target (on the point of impact) and the somewhat larger box is open to receive the smaller one. The two are seperated by a piece of paper or parchment (rice paper is prefered). When the missile is fired, the casing will crush upon impact, forcing the smaller ExDim box into the larger ExDim box, resulting in an extradimensional rift.
Roll 1d10 on a successful hit
1-3 Device malfunctioned (dud), roll regular ballista damage.
4-8 Missile explodes violently, causing additional 2-4 hull
points and 4d10 hit points damage to anyone within 20’
9-10 Rift created, everything within 10’ radius is sucked into
the Astral plane, ship takes Hull Holed and random Critical
Hit. Crew or weapon results mean anihilation via the rift.