What is a Martial Summoning Device?
A summoning device is any magic item with a summoning spell/summon entity bonded to it. In the martial context it specifically refers to weapons, shields, and armors that have been thusly enchanted. Martial devices are much less effective than normal magic items as they are made for non-magic users to have access to, and the magic has to be adapted for those limitations.
A summon spell only has a 25% chance of activating per scene/combat engagement. This is not cumulative, and if unneeded, the summon cannot be saved for later.
The summon creature appears at an encounter level equal to the level of the summoner
The summon creature spell is not random, when activated, it is always the same magic creature brought forth. If this is any sort of danger by proximity creature, like a fire elemental, storm giant, poison cloud of flies, the summoner has no special protection from their proximity.
The summon spell remains active for 1 combat turn, with the summon creature slotting into the initiative line-up, executing whatever it's attack plan is, and then dissipates.
Creating a summoning device is a basic enchantment ritual, but requires bargaining with the entity that is going to be summoned. For normal magic items this isn't the case, but such items can generally only be used by someone with magic potential. In the case of the martial summoning, the spirit will be providing the power rather than the summoner. This is the main cause of the fickle nature of the device and it's operation.
Who Makes Martial Summoning Devices?
Wizards and Sorcerers - to equip their champions and heros, giving a surprise magic attack capability to normally mundane combat damage dealers.
Necromancers - to give their evil lieutenants the ability to channel the power of their evil monsters into battle.
Cultists - because summoning aberrations and abominations is taxing, but a minion with a magic sword is much safer than the cult leader summoning a chaos beast, especially for the cult leader.
Healers - defensive summons can allow a healer respite to work on an injured person, or even cast healing magic on their behalf.
Cursed Items - because a magic sword is too good to pass up, and it only summons a hostile monster to attack the swordbearer 25% of the time.
Martial Magic Items in the Hands of Magic Users
Most martial magic items are made from the sort of gear that wizards, sorcerers, and magi generally don't or in game terms, aren't allowed to equip. It doesn't prevent your warrior/mage power gamers, or more hearty sword swinging Gandalf types from using said items. Being a magic user and having the ability to fully understand the power of the weapon, they can chose to expend a spell ability and activate the martial magic item without relying on the 25% chance of summoning. They are also not at risk of having the summon spell activate without their wish. When summoned, the magic creature behaves as a if it were summoned via a normal spell, and has longer duration, and can be guided by the summoner.
While wizards and other bookish magic users are not fond of these items, there is no reason that they would not be wildly popular among the demi-magic user community of red mages, bards, druids, and other borderline non-grimoire toting but still magic using crowd.
Point of Origin
In the game Final Fantasy IV summon spells were going to have the option to bond with/come from weapons, and there was a 25% chance that they would be randomly summoned in battle. This is perfectly fine in the confines of a linear JRPG, but when ported over into a traditional D&D setting it becomes a lot more interesting and exciting. What is the defining point in battle for the summon to activate? Is drawing the weapon enough, I hope not because a character could flourish their blade in a practice duel and suddenly there is Bahamut lord of the Dragons blasting the ever loving hell out of the practice pitch. Or, a character accidentally pulls a weapon, in a cramped confined, flammable space like inside a wooden bar, and then Ifrit is standing, boiling with flame, and setting the entire inn on fire. That's not gone well. The martial magic item does this. They have to be used carefully, and judiciously. Summon spells have collateral damage, proximity effects, and they are uncontrolled. The summoned being doesn't know and doesn't care about the difference between a sparring duel, a minor encounter, or a boss fight. When it appears, it appears ready to throw down as hard as it can for one brutal round of combat.