Magic to hit; whirlwind
Magic to hit; battering ram
Magic to hit; fire attack
Magic to hit; water domain
Elementals: Elementals can only be hit by those wielding magic attacks (any hero listed qualifies). Air elementals can form a whirlwind each turn and automatically eliminate one 1HD-figure. Earth elementals do damage to castles and other structures. Fire elementals do flame damage (e.g., deadly to trolls). Water elementals outside a stream or pond are reduced to 6" move and 1 damage.
Additional notes: The statistics above assume elementals of either 16HD (conjured), or possibly 12HD (device) size. While totally invulnerable to any normal troops, it's accurate to allow any heroes to hit them (big monsters like dragons & giants can hit them in any of Chainmail, OD&D, and AD&D, under a variety of different text language). There may be other restrictions, possibly dependent on your particular ruleset (like earth and fire elementals not crossing water, per OD&D Vol-3 p. 18), which can be adjudicated at your discretion. Note that no cost is given to elementals by themselves (the same as in Chainmail p. 36; see more below).
Why was the decision made to leave elementals out of the published book? Early play-testers pointed out a couple ways in which they were problematic (especially, my very perceptive friend Paul). In particular: (a) Conjure elemental is a 5th-level spell, and thus missing from BOW, which focuses only on top-level (6th) spells. I was instead giving elementals a fixed cost, and it was pretty dissonant to have them in the game but not summoned by wizards. (b) More generally, with elementals able to wade through any standard troops and completely ignore their attacks, pricing them was very much guesswork (I've estimated them at about value 40 each?). I became worried that in the absence of an opposing hero-type (esp., a wizard or dragon), they would be unbalanced and wind up breaking the game. (c) Earth elementals have the special "super battering ram against walls" power, but I didn't include any wall-destruction rules in BOW, so this particular rule was unsatisfyingly vague. Stuff like that; so in the end it seemed best to remove them and give a fuller treatment of options as an expansion option (with castle destruction yet to come).
So let's fix this now: here's conjure elemental as an optional spell for wizards in BOW:
- Conjure Elemental (Range: 24 inches, Duration: Concentration). This spell summons an elemental of any one type. It appears within range from a large body of the appropriate elemental material. If concentration is lost (the wizard moves, casts another spell, or is the target of any feasible attack), then the elemental switches sides -- but assume the wizard can dispel the elemental if needed (as a half-move action). At most, one elemental of each type can be conjured per day.
Other notes: The fact that the elemental locks down the casting wizard from any other movement or action, by concentration, is a big balancing factor (thanks, Paul!), and you may consider giving this power to wizards, in addition to those already specified in the book, for free. However, I'm a big fan of charging for any increase in options/flexibility, so I would add +5 to the price of a wizard with this spell available. If you further introduce a full complement of 5th-level spells (around 4 for any BOW-level wizards: perhaps conjure elemental, animate dead, transmute rock to mud, and cloudkill), then I would suggest maybe a total +10 price increase for that. However, granted how long a standard game lasts, it's borderline unlikely that a wizard will actually get to use any of these lower-level spells.
In addition, I would be strict about the need for "a large body of the appropriate elemental material". This is generally trivial for air and earth elementals. Water elementals should be limited to a "pond, stream", etc. [OD&D Vol-2, p. 19 -- also the last two terrain types in BOW]. Fire elementals will be more troublesome -- either a large bonfire must be set in advance, or perhaps fireballs can set an area of woods on fire, etc.
I'd recommend that concentration be lost as soon as any attack is launched at the wizard that has a possibility for success -- this from closely reading text for elemental monsters like "being attacked... will tend to break this concentration" [OD&D Vol-2, p. 19], as well as similar text in 1E, etc. This also simplifies the abstraction at our chosen scale. However, if you prefer to have concentration lost only from a successful attack, then be sure to switch to Special Combat (1-to-1) for any such concentrating wizard!
One final thing (I think): This is rather dissimilar to how I presented conjure elemental in my earlier OGL Book of Spells. There, I downplayed the ongoing concentration requirement; had a very short duration of 3 turns (translated from 3E); and also required a single control check at the very start of the spell. This latter I was thinking to fill in for the automatic AD&D 5% chance to lose control every round (see insightful comments by Jerry here) -- and for what it's worth, the exact language was copied from 3E's pipes of the sewers (which is possibly the only piece of text remaining in 3E with a chance for a PC power to backlash against them).
What I didn't realize until I recently went through the copious text on elementals was how unusual that roll-for-loss-of-control is, if we compare different D&D editions (it's only in 1E & 2E). Now I see that the real common core of the spell is (a) the daily type limitation, (b) need for concentration, and (c) raw material requirement. At some point I'll get around to updating Book of Spells, and that's one of the adjustments I plan to make, so as to bring the spell more in line with classic D&D.