smaller creatures fighting ON larger ones...

[Amazing Mumford]

Any specific rules about, say, a small-size character leaping onto the back of a large (or bigger) monster, like a halfling with a dagger trying to jump onto a frost giant? And trying to stay on and fight it from that position? What about like a human leaping onto the back of a huge dragon who was 10' or so below? Does grapple really cover this? Not saying whether it's a smart or dumb move, just how it would work? Any penalties for the larger creature trying to reach the smaller one to strike it?

[Alzrius]

This issue is not covered in the Core Rules at all.

While you could try and extrapolate some rules from existing mechanics for things like Jump and Grapple, you'll have your work cut out for you. The best mechanics I've ever found in this were in an article in Kobold Quarterly #3 - I recommend just buying the issue, as it has this and quite a bit of other great material, but I'll summarize its rules here.

The article mentions some specifics about actually getting onto a creature (it must be two size categories larger than you) which are mostly about making a Climb (from below) or Jump (from above) check to get on it, with a Tumble check to avoid an AoO.

Once you're on the creature, you gain a +2 to attack rolls against it, it's also flat-footed against you, and your critical threats against it are automatically critical hits. However, there are some penalties for you as well. You can't make a full-attack action against the creature, you need to make a Balance check (opposed by the creature's Strength check) on both your turn and its turn each round to stay on it's back, and if it succeeds, you fall off of it (check for falling damage) and land prone in an adjacent square.

[Jeff Wilder]

Originally Posted by Alzrius

This issue is not covered in the Core Rules at all.

Well, almost not at all. In addition to the Grapple rules, in which two or more creatures can occupy the same space, there's the below.

From the SRD:

Very Small Creature

A Fine, Diminutive, or Tiny creature can move into or through an occupied square. The creature provokes attacks of opportunity when doing so.

Square Occupied by Creature Three Sizes Larger or Smaller

Any creature can move through a square occupied by a creature three size categories larger than it is.[quote]It'd obviously be a house rule, but you could extrapolate from the first rule, and change the second to allow "occupying" a square as well as moving through it. Just be aware that you're getting into some weird areas that you'll most likely have to adjudicate on the fly, like, say, "When does the smaller creature flank, and with whom?"

[Shin Okada]

Not jumping onto. But with Giantbane tactical feat in Complete Warrior, you can climb up the body of a creature 2　or more size category larger than you.

[irdeggman]

Yes grapple is the mechanci to use.

Remember that D&D combat is by design vague.

Also that there is no facing (using non-optional rules) so in all of the examples you bring up an AoO is granted since the smaller creature is always moving into the same square as the larger one (hence leaving a threatened square and not using a 5 ft step).

[Thanael]

Originally Posted by Shin Okada

Not jumping onto. But with Giantbane tactical feat in Complete Warrior, you can climb up the body of a creature 2　or more size category larger than you.

Here's the full text of that feat: Feats - Giantbane

[Amazing Mumford]

Hmmm, I guess the Giantbane Feat does pretty much cover the basics... it seems a little restrictive that you would need to use a feat to attempt this though. I mean, you see it in movies quite a bit! (I know, I know, this isn't a movie...) There are some situations where this could be useful though... although winning an opposed grapple with a Cloud Giant or Huge Dragon would be tough in and of itself! If you do happen to be on the back of the dragon, at least you can't get full-attacked by it! Does anyone have any suggestions on maybe imposing penalties to characters who try this without the feat? Like you can attack with a weapon in each hand if you don't have Two-Weapon Fighting, but there are penalties involved...

And Alzrius, thanks for the Kobold Quarterly suggestion, by the way... Very interesting, but we as a gaming group have so many WotC books and Dragon and Dungeon magazines that sometimes hunting through what we already have is very time-consuming...

[Shin Okada]

If the designers of 3e tried to incorporate all the possible "moves which fantasy heroes may do" into the combat system itself, "Combat" section of PHB would have been a one huge book by itself. Or maybe it occupied 2 or 3 core rule books.

Instead, they made many such maneuvers into feats and class features.

So, while the combat section of PHB is reasonably short, when you really want to shoot down opponent's weapon as many heroic gunman are doing, you can take Ranged Disarm feat in CW.

But it is also true that DM should better avoid simply say "No". So, I guess you can take somewhat reverse approach.

In case of maneuvers which is already covered by the core rule, specific feat often gives the PC some benefits, like,

Change the required action type (Improved Feint)
Remove AoO (many of the PHB Improved XXX feats)
Add +4 or so of Bonus (many of the PHB Improved XXX feats)
and such,

So, you may do the reverse, Climb Aboard maneuver is a free action & requires a DC 10 Climb check. And the opponent can try to shake the PC off by grapple check vs Climb Check.

So, when a character without this feat tries to do the same maneuver, you can say, for example,

It requires a standard action
This maneuver itself (in addition to the movement) provokes an AoO
The character must beat DC 14 Climb Check
Opponent can shake the character off by making a grapple check vs Climb check -4

I am not sure if this is enough to make it balanced. I guess it will need a lot of testing if you continue to allow it in your games. But when a player wants to do something not covered by the rules and DM must adjudicate on the fly, such approach may work.

[Amazing Mumford]

Hey Shin, thanks for the suggestions-- the combat coverage issue goes without saying, it would be nearly impossible to cover everything! Our group does tend to adjudicate on the fly, and in the time between sessions formulate a solid house rule. I like the "standard action" and "climb check" suggestions, though I might make the climb check DC a little tougher to simulate the foe not being a totally stationary surface...

There's just too many feats to choose from in the darn game!! And a lot of those cool prestige classes and feat trees don't give you much wiggle room! Oh well, for the sake of balancing...