In my DF game, we changed around explosive spells a few times until we settled on something we liked. The following is how we run the spells. I can vouch for it being fast, easy, and free from argument and problems.
Explosive Spells: There are no specific explosive spells. All missiles spells which create their own projectile can be cast as an explosive version at double cost. The caster must decide before making the initial spell roll if the missile is explosive or not.
Spells which are specifically a ray (Sunbolt) or which use ammunition (Poltergeist, Winged Knife) cannot be made into explosive spells.
This was previously written up in this post.
Explosive Diameter: All explosive spells cover a 5-yard diameter, regardless of the energy placed into it. Damage is full on impact and in the center hex, 2/3 in the ring of hexes around that, and 1/3 in the outer ring of hexes. Roll damage only once and divide. Spells may be targeted at the ground in a specific hex, at +4 to hit, but misses are subject to Scatter (p. B414) and Hitting the Wrong Target (p. B389), as appropriate. (In other words, if you miss, you may toss it long or short, but if you had friends in between you and the target hex, you might hit them directly.)
This was previously described in this post.
Some exceptions/rulings/FAQs for my Dungeon Fantasy game:
- If a caster's spell list includes either Fireball or Lightning but does not include Explosive Fireball or Explosive Lightning, then they are not able to cast the explosive version of the spell. For Example: Druids have Lightning as a PI4 spell. They do not gave Explosive Lightning at all. Therefore they can learn Lightning but cannot cast the explosive variation. In cases where there is no Explosive variation to compare to, the GM makes the call.
Why? Same reason clerics can cast Stone to Flesh but not Flesh to Stone even after the spells were merged - it's a specific carve-out, and I want to retain that. Druids, for example, are nature priests, not artillery pieces (a wizard niche), so they only get to toss lightning not blow up whole areas.
- Just a FAQ answer: No, you cannot "airburst" explosive spells. You generally cannot aim them in such a way as to hit the hex of a foe and have the foe block your close-by friends. That may happen automatically with large foes (see Combat Writ Large, p. 5, Pyramid 3/77) but it can't easily be finagled with aim.
Why? It's come up where people want to, say, target someone with an explosive fireball and hit him and his surrounding friends, but not a friendly in the blast radius. To avoid long discussions about "if I hit his shield side, wouldn't that mean my buddy is shielded by his body and take less damage?" and so on, we treat it basically as a thrown, short-duration, diminishing-effect Area spell that radiates out from a center point. Sufficiently large obstacles (and yes, foes of the right shape) can occlude areas.
- Explosive spells that impact on a target preferentially damage that location even when inflicting Large Area Injury (p. B400). This can matter for crippling injury, stunning, etc. IOW, if you hit a specific location, it will take damage (DR permitting), but it is generally better to just hit the area.
- Explosive missile spells do not automatically fill the volume of a 5-yard sphere - walls, solid obstacles, etc. stop the expansion of the blast. It's magic, not physics, and damage is spraying out not being created to fill an area with uniform density.
- They also ignore most of the usual "explosion" rules - fragmentation, damage divisors by distance, etc. It's not an explosion per se, it's an area-filling missile spell with diminished effects further out from the center. Diffuse creatures are still affected as if this was an "Area effect, cone, or explosion" attack.
That's a lot of little rulings, but it takes more time to write them then to follow them in play.
- While full cover does block an explosive spell, it has to be strong enough to resist the spell (sliding paper doors, for example, wouldn't help much), otherwise it merely acts as DR. A target's body doesn't provide cover unless it completely covers the potentially shielded victim. For example, a child or pet covered by a prone adult, or anything beyond a dragon larger than the radius of the fireball so nothing can "leak" past. Generally, assume everything in the covered hexes gets hit.