I do not recall spells exploding in TES oblivion where you wanted them to, instead they went in a straight line untill they hit something.
Well anyway, you simply have to create a unit collision detection system by using the unit enters X distance of unit event for efficency or a generic pick of all uints near by, which i do not recommend since it is too unefficent.
remember when using that event that there is no function to get the unit which distance was enterd of (the unit you gave for the event) and so you will eithor have to use a struct system or a chache system like handle vars which you attach the center unit to the trigger and so you can retrieve it appon a detected collision.
For movement you can eithor use the standard WC3 move which can bug, a trigger based movement system which is more precess intensive or a basic terane phisics enhanced movement system that is recomended for this kind of spell as to stop the bolt suddenly flying up or off cliffs and remaining beautifully the same ammount above the ground which TES oblivion did not have. If you have studied phisics and have decent know how of maths such height systems are easy since you can get the terane height and the projectile flying height and add them to get the total height which the bullet has to mantain or in the case of a hill / cliff explode at (if the total height is less than the terane height). It does get complex though when you are dealing with firing from a cliff or elevation since you have to include Z rotation of the projectile and losing/gaining height as it moves as to stop it being tedious and hard to aim and hit stuff.
For the Z rotation you can do the easier method of simply reducing or increasing the total height by a culculated ammount by compairing the height of the target location and the location of the caster which yeld the same results as a Z rotation but more efficently as you do not have to constatly preform advanced trigometrical culculations.
