In the Arkham Horror base game, the spell Enchant Weapon has the ability to make a Physical Weapon into a Magical Weapon until the end of this combat (assuming that one such combat exists). Similarly, in the Arkham Horror expansion Lurker in the Threshold, the spell Intervene has the ability to +3 to a combat check when cast before that investigator makes the check (again, assuming that one such combat check exists). Can these two spells be used for helping other investigators?
In the card game Magic:The Gathering, targets are defined in a way to promote ingenious play, such that creative uses for spells exist (e.g. Denial Bazaar of Baghdad + Balance). Assuming that the targets in Arkham Horror are defined in the same way (so as to promote emergent gameplay), the target for Enchant Weapon can be any weapon, regardless of who owns it. That means the spell can be used to target the weapon of another investigator, regardless of the distance between the spellcaster and the other investigator, so long as that investigator is in combat.
For example, if Joe Diamond was facing a Physically Immune ghost monster with only his trusty peashooter in combat somewhere in Kingsport, Jenny Barnes in Innsmouth can cast the spell Enchant Weapon so that Joe's gun will actually damage that ghost until then end of his combat, even though neither the weapon nor the combat is hers.
For the spell Intervene, let's say that Joe Diamond decided to investigate the Graveyard and got this encounter:
Descending into a dark mausoleum, you discover a vampire rising to feed. You quickly find yourself fighting for your life. Make a Combat (-2) check. If you pass, you defeat the vampire, gaining 1 Clue token and drawing 1 Unique Item. If you fail, roll a die and lose that much Stamina.
If Jenny Barnes in Innsmouth had the spell Intervene, then she can THEORETICALLY use the spell to give Joe Diamond a +3 Combat check modifier to help him out, even though she is on another board. Since the set of valid targets for Intervene is any investigator, and Joe is an investigator (and therefore an element of that set), then Joe should be able to reap the benefits of a successfully cast Intervene even though he isn't the one casting it.
The fact that Jenny Barnes uses up two hands to cast Intervene is irrelevant, since she can simply release the spell after Joe's check. The spell only lasts for ONE combat check, and she can immediately free up her hand resources as soon as the spell's duration is done, with no lasting effects.
Same thing with the spell Mists of Releh and Bind Monster. Mists of Releh's target is an Evade Check, which means ANY Evade Check, not necessarily the caster's. Bind Monster's target is a Combat Check, which means ANY Combat Check, not necessarily the caster's.
Since the Mists of Releh and Bind Monster's targets are skill checks, and the set of targets have no restrictions on who is actually performing those checks according to the game's rules, then the Mists of Releh and Bind Monster can be cast for checks made by other investigators, since checks made by other investigators are also elements in the set of targets.
The fact that Bind Monster uses up two hands for casting is irrelevant when it is used for another investigator's combat check, since the caster isn't actually participating in the combat check and wouldn't need the hands for anything else. The only relevant factors would then be spell check success and sanity cost for the spellcaster.
I may be mistaken in case there has been an official ruling that I don't know about as to the timing of these effects. I would have to assume that these spells can be cast at any time, for any valid target, much like Fast Effects in the game Magic:The Gathering, since I haven't come across any restrictions regarding targeting and timing.
Such card play can be used for greater cooperation between investigators, and I would like to know whether these applications are valid for when I need to unleash a killer card combo against awful monsters, such as Dholes.
To recap: Can an investigator benefit from the spells Enchant Weapon, Intervene, Mists of Releh, Bind Monster, et al, cast by another investigator given that the targets and timing are valid?