Phantasmal Forces: The creation of vivid illusions of nearly anything the user envisions (a projected mental image so to speak). As long as the caster concentrates on the spell, the illusion will continue unless touched by some living creature, so there is no limit on duration, per se. Damage caused to viewers of a Phantasmal Force will be real if the illusion is believed to be real. Range: 24".The range for both is the same, and could be redefined as "line of sight", in the scheme I've been presenting. It's a little odd that you would need a 240-foot range for Invisibility, but I suppose you could cast it on a scout creating a diversion after the diversion, to enable the scout to escape. Or vanish a bridge before an enemy rushes across it to attack.
Invisibility: A spell which lasts until it is broken by the user or by some outside force (remember that as in CHAINMAIL, a character cannot remain invisible and attack). It affects only the person or thing upon whom or which it is cast. Range: 24"
Phantasmal Forces, as has been pointed out, gets its name from its use in Chainmail to create a military force of phantoms. This is in contrast to some later versions of the spell that specify that it must be an illusion of a single object or creature.
This restriction is usually added to prevent the creation of things like phantom bowmen: illusory arrows are distinct from the illusory archer firing them, so they are forbidden. But the specification of damage being real if believed, in contrast to the rule about illusions disappearing when touched by a living being, suggests that a squad of phantom bowmen firing arrows at the enemy may, in fact, be legal. What the Magic-User can't do is create a squad of hidden or invisible bowmen.
Disbelief is a whole other kettle of worms, which has been discussed to death. I posted previously about disbelieving illusions (and incidentally came down against phantom bowmen, but I've softened on that stance.) The gist: disbelief doesn't dispel the illusion, but it prevents damage, and it's automatic if the player can cite a reason why they don't believe. To that, I'd maybe add a chance to notice an inaccuracy in the illusion if the PC's Int or Wis is > the M-U's Int/Wis.
Phantasmal Forces is the first concentration spell, with an open-ended duration as long as the caster isn't disturbed, casts another spell, or performs another action. Perhaps we can consider this as a change of state spell, where the "state" is "the caster's ideas become manifest". When the idea changes drastically (caster starts thinking about something else, like "Ow! My arm!") it's like being shaken awake from a Sleep spell; the state changes again.
Change of state is going to be a critical concept for Invisibility as well. On first reading, Invisibility seems permanent. Certainly, if a treasure chest is made invisible, it could remain invisible forever, since the chest can't attack and you have to find it first before you can cast Dispel Magic on it. But there are a couple provisos on the spell.
- It can be broken by the user. The question here is: does "user" mean "caster", or does it mean "target"?
- It can be broken by an outside force. No explanation of what counts, unless "attacking" is somehow meant to be an example.
- It only affects the target of the spell.
The way I interpret the third restriction is that Invisibility, unlike Phantasmal Forces, only affects one thing. Clothing and gear are part of the character affected only as long as they remain stowed: weapons remain sheathed, items remain stowed in a belt or pack. Any "splitting" of the invisible object changes the state of the object, and the invisibility fades. Likewise, taking off or putting on clothing ends invisibility, as does eating or drinking (perhaps with a brief, disturbing moment where the food or liquid is the only thing visible...) Similarly, splattering or spraying liquid, dust, or other material on an invisible opponent ends the spell (these are the "outside forces".) This makes the spell open-ended, but not permanent.