To counter a spell, expend one prepared spell and follow the formula presented for dispell magic:

Dispell Magic: Unless countered, this spell will be effective in dispelling enchantments of most kinds (referee’s option), except those on magical items and the like. This is modified by the following formula. The success of a Dispell Magic spell is a ratio of the dispeller over the original spell caster, so if a 5th level Magic-User attempts to dispell the spell of a 10th level Magic-User there is a 50% chance of success. Duration: 1 turn. Range: 12″.

Source: Men & Magic, page 25.

That is, any prepared spell can be “converted” into an antagonistic dispell magic. One could without issue probably extend that to full dispell magic functionality (thereby doing away with the need to find, learn, or prepare dispell magic as a distinct spell), given that removing enchantments is a relatively core aspect of wizarding, though I could understand wanting to keep the full dispell magic a separate thing.

Note that OD&D does this differently than AD&D, which works like the BRP resistance table (50% success modified by level differential). To make the difference clear, in OD&D a 9th level magic-user has a 90% chance of successfully dispelling an 10th level magic-user’s enchantment (9/10), while in AD&D the chance would be only 45% (50% -5% for having one less level).

Alternatively, compare spell levels rather than class levels using the OD&D formula. So, expending a second level spell in an attempt to counter a third level spell would have a 2/3 (66%) chance of success. This method might be preferred if you see the countering process as an opposition of specific spell energies rather than a contest between the overall skill of the two magic-users.

The burn spells paradigm is becoming increasingly attractive to me. There is some danger of complexity creep, so the possibilities should be limited to a small number of effects. That said, having several default options frees up magic-users to prepare more obscure or interesting spells, which otherwise might not get as much use, much as 3E clerics were able to convert any spells into cures. Further, requiring the expenditure of prepared spells to power such effects retains some degree of resource constraint, unlike make other unlimited or at-will approaches, and doesn’t require tracking any additional information, since spell slots are already managed.

The options so far that I have thought about are maleficence, magical defense, and now dispell magic. I could see adding read magic to that list perhaps, though I am also experimenting with replacing read magic with a “skill” type d6 roll that comes at the cost of an exploration turn.

(This idea came to me when I repurposed dispell magic for banishing summoned creatures.)

Mateo had the (brilliant) idea to use the retainer and morale system as the base for a summoner class. His approach adjusts the loyalty rolls based on summoner level, monster HD, and other factors. This allows higher level summoners to control more powerful creatures (on average).

Here is another take on the same basic idea, but using the retainer system literally. That is, summoned monsters just occupy retainer slots. Reaction, negotiation, and loyalty rolls are made exactly as if the monster was encountered in the wild. Base rules are 3 LBB OD&D, but I suspect a similar approach would work with other systems. See pages 12 and 13 in Men & Magic and also this old post I wrote about OD&D loyalty and morale.

Monsters defeated in combat may be subdued and brought into service following the rules in Men & Magic. Now, we could pretty much just end there and have a relatively comprehensive system. Magic-users would need to find minions during adventures and convince them to serve or employ charms. However, I like the idea of magic-users being able to gate in creatures directly, and the only even remotely similar spell in Men & Magic is conjure elemental. The monster summoning spells introduced later are not to my taste. So it seems like at least one new rule is required, in the form of a spell that any magic-user could employ if learned:

Summon Monster, level 1 magic-user spell

In a puff of smoke, a monster appears. Use the dungeon random encounter tables, determining first dungeon level* then monster with dice. Establish reaction normally, adjusting for incentives (2d6: 5- hostile, 9+ friendly). Only one monster is summoned. Do not use the number appearing value.

I like the minions that Mateo crafted, but also find the idea of just leveraging everything in the Monster Manuals compelling.

Beyond the new spell, the core of the summoner’s art lies in the skillful application of other standard spells.

Summoned creatures cannot cross a properly prepared circle of protection (requires salt and casting the spell protection from evil). Creatures may be summoned into or outside of such a circle, at the summoner’s discretion.

Other spells that a summoner may want to master: charm person, charm monster.

The dispel magic spell may be used to banish summoned creatures. The “original spell caster level” is the higher of summoner level and monster HD.

The polymorph others spell may be used in conjunction with flesh to stone (using special techniques, they may be cast together, though both must be prepared separately) to transmute minions into figurines, holding them in stasis until recalled using dispel magic (no check required, given that you are dispelling your own enchantment). Such figurines are significant items, and creatures that resist are permitted a single saving throw versus the combined polymorph/stone effect.

Thus, higher level magic-users become better summoners because they are more likely to have all the other spells and be able to set up the summon, ward, compel “combo” more easily and reliably than a lower level conjurer.

In OD&D, I allow magic-users of any level to create scrolls of any spell possessed, even if it is too high level to prepare normally, at the cost of 100 GP per spell level and one downtime action. This rule is based on the scroll creation system in Holmes. It essentially allows ritual casting of higher level spells given time and sufficient GP. Thus, with proper access to spell texts and resources, even a low level magic-user could perform a summoning, but it would be a long, expensive, arduous undertaking, and still entail significant risk.

* Roll 1d6 for dungeon level if using The Underworld & Wilderness Adventures (page 10). The AD&D Monster Manual II probably has the best dungeon encounter tables (page 133), though, so I might use that instead, in which case roll 1d10 for dungeon level. The MMII provides the chance of randomly getting, for example, a duke of hell.