It's tough to have good hallucination rules. The usual fallback are confusion spells, which are just a small table (d4) of options that include stabbing yourself, stabbing an ally, etc. That's more of a "do random stuff" effect and not much of a "perceive random stuff" which is sort of what hallucinations would do.
There are a couple of reasons why it's hard to write good hallucination rules.
1. It depends on making up random stuff. Random stuff is hard.
2. If you're rolling on a big table before you tell a player "you see a purple dragon in the tavern, what do you do?", then the player will metagame and realize that the purple dragon is a hallucination. They can't help it.
3. The game pretty much depends on describing scenes to the group, and then letting them act on it. Hallucinations mess with this simple, effective process.
So here's my attempt.
Whenever a player is HALLUCINATING, roll on this table whenever something significant happens. E.g. monsters attack, the PC enters a new area, an NPC begins saying important stuff, etc. Also whenever something happens that only the hallucinating PC witnesses.
1 Wrong tool / wrong verb.
2 Wrong target / wrong noun.
3 Misinterpret the situation.
4 See things that aren't there.
5-8 Nothing. Describe the scene naturally.
You can make this roll every round to constantly add hallucinations, or you can roll once at the start of the scene and just maintain the same ones.
IMPORTANT: This will only work if you describe the scene to the hallucinating PC first and make him decide what he wants to do. Only after the hallucinating PC chosen their actions do you describe the scene accurately to the rest of the group.
IMPORTANT: The hallucinating player may say, "well, I look at my fellow party members and see how they are reacting before I do anything". This is not a bad idea, but this delay will also (a) cost the hallucinating person their first turn in combat while they look at their companions, and (b) also gets it's own roll on the hallucination table.
Example: Encountering hostile goblins.
- Tool/Verb: "You see some goblins about to attack you with feather dusters."
- Target/Noun: "You see some orcs about to attack you."
- Misinterpret: "You see some goblins having a nice tea party."
- Stuff that isn't there:"You see some goblins about to attack you. They have a troll with them."
Example: Seeing a waterfall.
- Tool/Verb: "You see a waterfall, except it's not falling. It's just standing still."
- Target/Noun: "You see a flow of gold coins pouring off a ledge and disappearing into the water."
- Misinterpret: "You see a cave with a waterfall. The air is dry and hot, and you can taste copper."
- Stuff that isn't there: "You see a waterfall with nymphs washing their hair beneath it."
Example: Hallucinations begin mid-combat
- Tool/Verb: "You thought you shot an arrow, but actually you just threw your sword."
- Target/Noun: "You tried to attack the ogre, but you accidentally attacked your friend."
- Misinterpret: "You thought you killed the ogre last round, but he's still here somehow."
- Stuff that isn't there: "One of the ogres opens his mouth and vomits out a grey ooze."
Each of these gives the players an honest choice based on dishonest information. That's good--choices are more fun than losing control of your character while the DM rolls his actions.
If you want a more granular mechanic, you can also roll a d4 to see how believable it is.
1 Completely silly. Goblins having a teaparty.
2-3 Something in between.
4 Completely possible. Goblins have a troll with them.
If you want a random noun generator to help you come up with stuff, just google one. I liked this one:
NOTE: In order to prevent players from metagaming, be sure to roll every time and present the information in the same way. Think poker face.
NOTE: If a PC is hallucinating, lay it on thick! So far I've only described the big, mechanically significant hallucinations, but you should also be adding in little fluff hallucinations constantly. "Fluff" hallucinations are the ones that still sound crazy but don't really affect the game much. Don't bother interrupting the flow of the game for these, because they won't have much gameplay impact (either because they're minor details that don't change how a player would make decisions, or the PC has time to look at how his companions are reacting).
Examples of fluff hallucinations:
1 Your spaghetti is writhing like worms.
2 You hear your father's voice, calling you to come home.
3 You can hear the sun burning in the sky like a bonfire.
4 Everyone in this marketplace is looking at your butt.
5 Something is growing inside your stomach.
NOTE: Test for false positives. Whenever something could happen that only the hallucinating PC could witness, roll the d8. Like when the waitress served the spaghetti, did she really whisper "meet me in the kitchen in five minutes" or not?
It's not a complicated rule, but it does take a while to understand. Rolling a d8 doesn't take much time, especially if you roll the d4 at the same time. Once you get the hang of it, it'll flow a lot smoother.
Honest choice. Dishonest information.