What combination of initiative rolls need to occur to disrupt spellcasting with a melee attack

The rules (such as they are) are on p.65 of the DMG

SPELL CASTING DURING MELEE

These functions are fully detailed in PLAYERS HANDBOOK. Their commencement is dictated by initiative determination as with other attack forms, but their culmination is subject to the stated casting time. Both commencement and/or completion can occur simultaneously with missile discharge, magical device attacks, and/or turning undead. Being struck by something during casting will spoil the spell.

Spell-casters will always insist that they are able to use their powers during combat melee. The DM must adjudicate the success of such use. Consider this: The somatic (movement) portions of a spell must be begun and completed without interruption in a clean, smooth motion. The spell as a whole must be continuous and uninterrupted from beginning to end. Once interrupted, for any reason whatsoever, the spell is spoiled and lost (just as if used). Spells cannot be cast while violently moving - such as running, dodging a blow, or even walking normally. They are interrupted by a successful hit - be it blow, missile, or appropriate spell (not saved against or saveable against).

Thus, casting a spell requires that a figure be relatively motionless and concentrating on the effort during the entire course of uninterrupted cast ing. For example, a magic-user casting a fireboll must be in sight of the intended area of effect during the course of the spell (although an associate could be there to open a door intervening between caster and target area at an appropriate time - provided the timing was correct, of course). The caster cannot begin a spell, interrupt it just prior to completion, run to a different area, and then complete the spell; inter- ruption instantly cancels it. Unless a spell has no somatic components, the caster cannot be crouching, let alone prone, during costing.

It can thus be understood that spell casting during a melee can be a tricky business, for a mere shove at any time can spoil the dweomer! Any spell can be attempted, but success is likely to be uncertain. Use the following procedure for spells cast during melee:

1. Spell casters must note what spell they intend to cast at the beginning of each round prior to any knowledge of which side has initiative.

2. Attacks directed at spell casters will come on that segment of the round shown on the opponent's or on their own side's initiative die, which- ever is applicable. (If the spell caster's side won the initiative with a roll of 5, the attack must come then, not on the opponent's losing roll of 4 or less.) Thus, all such attacks will occur on the 1st-6th segments of the round.

3. Intelligent monsters able to recognize the danger of spells will direct attacks against spell casters if not engaged by other opponents so as to be prevented from so doing.

4. The spell caster cannot use his or her dexterity bonus to avoid being hit during spell casting; doing so interrupts the spell.

5. Any successful attack, or non-saved-against attack upon the spell caster interrupts the spell.

Because spell casting will be so difficult, most magic-users and clerics will opt to use magical devices whenever possible in melee, if they are wise.

The reference to the PLAYERS HANDBOOK is not material but its discussion of Initiative begins on p.104 and goes through to the Example of Combat on p.105.

Note that AD&D rules inconsistently use the word MELEE - sometimes it means opponents in close-quarters engaged with melee weapons, other times it is used as a synonym for combat. The Example of Combat makes it clear that in the instance of SPELL CASTING DURING MELEE it means combat in general.

The important point above is No.2 which is bloody hard to parse. It is also flatly contradicted by what is said on p.66, which we'll get back to.

It says, if the spellcaster won initiative, the attack comes on the segment of the winner's initiative roll. This seems to imply that if the spellcaster lost initiative, attacks come on their losing roll. If this is correct, then attacks happen early in the round: the median is 3 and the mean is 3.08.

However, it's possible that this means it would always come on the winner's roll. If so, Mr Gygax could have just said that, however, while a man of many talents, clarity of writing was not one of them. If this is correct, then attacks come relatively late: the median is 5 and the mean is 4.47. This would make casting short spells a relatively safe bet but since this goes against the whole tone of the section, I don't think that's what it means.

How this works when one side is surprised is anyone's guess because "Surprise gives initiative to the non- or less-surprised party" meaning you don't roll dice for the first round.

The other rules are on p.66

Other Weapon Factor Determinants: The speed factor of a weapon also determines when the weapon strikes during the course of the round with respect to opponents who are engaged in activity other than striking blows. Thus, suppose side A, which has achieved initiative (action) for the round, has a magic-user engaged in casting a spell. Compare the speed factor of the weapon with the number of segments which the spell will require to cast to determine if the spell or the weapon will be cast/strike first, subtracting the losing die roll on the initiative die roll from the weapon factor and treating negative results as positive. Example: A sword with a factor of 5 (broad or long) is being used by an opponent of a magic- user attempting to cast a fireball spell (3 segment casting time). If the sword-wielding attacker was represented by a losing initiative die roll of 1, the spell will be cast prior to the sword's blow. A 2 will indicate that the spell and the blow are completed simultaneously. A 3-5 will indicate that the blow has a chance of striking (if a successful "to hit" roll is made) before the spell is cast, arriving either as the spell is begun or during the first segment of its casting. Suppose instead that a dagger were being employed. It has a speed factor of only 2, so it will strike prior to spell completion if the initiative roll which lost was 1-4 (the adjusted segment indicator being 1, 0, 1, 2 respectively) and simultaneously if the die score was a 5. If the weapon being employed was a two-handed sword (or any other weapon with a speed factor of 10, or 9 for thut matter) there would be no chance far the reacting side to strike the spell caster prior to completion of the fireball. Note that even though a spell takes but 1 segment to complete, this is 6 seconds, and during that period a reacting attacker might be able to attack the magic-user or other spell caster prior to actual completion of the spell! If combat is simultaneous, there is no modification of the weapon speed factor.

So, that's clear, right?

Maybe I can put it better:

• If the spellcaster lost initiative, their spell will always go after the actions of the winning side and any "blow, missile, or appropriate spell (not saved against or saveable against)" will disrupt the spell.
• If initiative is tied, then the spell goes first if its casting time is less than the weapon's speed factor, simultaneously if they are the same and after if it is larger.
• If the spellcaster won the initiative, you subtract the losing initiative roll from the weapon's speed factor and take the absolute value of the result. If that is less than the spell's casting time, the spell can be disrupted, if its the same, the blow and the spell go off simultaneously and if its bigger, the spell goes first.

What we did

I cut my teeth on AD&D and we didn't use either of these incompatable rules. If you won initiative, you could safely cast your spell, if you tied it would be cast as you got hit and if you lost then the other side went first and you ran the risk of losing your spell.

Remeber, initiative is turn-by-turn in AD&D and actions must be declared before you roll. We didn't do that either.