Here we are at the final (at least for now) installment of my little series on the B/X spell lists. Of course there are several more levels of spells, but the first three are the ones that come into play by far the most often.
9. Lightning Bolt: The other iconic "big gun" spell, after Fire Ball. Besides the difference in damage types (which rarely comes into play in B/X anyway, except for a few creatures with vulnerabilities to fire or cold) the major difference is in range and area of effect. A Lightning Bolt can start up to 180' away, and projects a bolt 60' long and 5' wide. Both inflict 1d6 points of damage per level of the caster, but for the math geeks out there, a Fire Ball affects an area of about 1,256 square feet (pi times 20' radius squared) while Lightning Bolt affects a measly 300 square feet (60 times 5), making Fire Ball the better spell by far in terms of pure damage potential. Fiction-wise, though, I always thought it was cooler to throw lightning than fire, but that's a matter of taste.
Lightning Bolt is also a lot more hazardous in confined spaces: if the bolt strikes a solid surface such as a wall before reaching its full 60' length, it rebounds back toward the caster until the difference is made up. I seem to recall some edition in which a rebounding bolt could inflict damage both on the initial incidence and the reflection, should a target be unfortunate enough to get caught in both, but in B/X this isn't stated to be the case.
A DM with a bare minimum of knowledge in physics and geometry could easily modify the reflection rule, applying the physical law that angle of incidence equals angle of reflection, and allow the caster to pull off some cool bank shots, and even have the bolt ricochet more than once.
10. Protection From Evil 10' Radius: Take everything I love about the 1st level Protection from Evil spell and extend it in a 10' radius around the caster and you've got this little gem. Cast this, and the entire party can cross a room full of vampires, gargoyles, elementals, or any other enchanted creatures completely unscathed. Oh, it offers the trifling little bonuses of +1 to saving throws and -1 to the attack rolls of such creatures as well, but that's a side benefit at best. It also has double the duration of the base spell, lasting a full 12 turns.
The spell description doesn't state whether a protected individual attacking an enchanted creature breaks the barrier for the entire group or just that character. (The Mentzer rules offer clarification: any protected creature attacking will negate the barrier for all those protected with respect to the specific creature attacked but not to others. Thus, if the fighter attacks a gargoyle, the creature is now free to swoop in on anyone within the circle of protection, but a specter that hasn't been attacked by anyone is still blocked. It makes perfect sense to me, but if you're a B/X purist, that's only one possible interpretation and not gospel.)
Also left unspecified is whether a character stepping outside the circle of protection can regain it by re-entering the circle. Mentzer offers no advice here either. Personally, I'd rule that the protected area is protected, period, unless the caster personally breaks it. Step outside, and you're vulnerable; step back in, and you're protected again, with the caveat that any creature you attacked while outside is now able to enter the barrier.
11. Protection From Normal Missiles: There are all sorts of tactical reasons why being completely immune to arrows, sling stones, and thrown weapons would prove useful. It can enable a thief to climb a wall without being picked off, a magic-user to fly or levitate above the field of battle with almost complete invulnerability, or an archer to stand in the open and rain his own volleys of missiles on the enemy. A captain could make a taunting speech from the battlements, a la Aragorn at Helm's Deep, amid a hail of arrows.
I would think that trap-fired projectiles such as poison darts would also be among the sorts of missiles the spell deflects, so it's even of some use during a dungeon crawl with little ranged combat. With a duration of 12 turns, you can get in a lot of exploring and fighting before it expires.
It won't block enchanted missiles, nor huge ones like catapult shot, ballista bolts, or giant-hurled boulders, but those are a distinct minority among the types of missile fire likely to be faced by adventuring PCs.
12. Water Breathing: This is one of my favorites for the simple reason that it grants characters an ability truly beyond ordinary human limits. It's one of the most quintessentially magical magic spells in the rules, in my estimation. It enables a character to breathe underwater without hindering the ability to breathe air, and it lasts a full day, making otherwise completely inaccessible places possible to explore. The only drawback, and it's a pretty significant one, is that it affects only one creature per casting. Even at the highest levels attainable in B/X, a magic-user can only cast it four times per day, so it's probably not going to enable an entire party to go adventuring beneath the waves. For a one or two person dive into a murky pool, an underground river, or a sunken shipwreck, though, it's perfect. It could prove helpful if you need to fake a death by drowning, and it might also come in handy in a pinch if someone in metal armor falls into deep water, since it has a range of 30'.
And with that, it's time to wrap up the Spell Roundup and move on to other topics.