Using a Huge or larger mount in combat

[kjenks]

We had some problems with an NPC villian mounted on the back of a gargantuan wyvern, who charged with a lance, at the last game I ran.

First, one player playing a druid/master of many forms tried to grapple the rider (the druid was not airbourne yet). I overrulled the grapple, saying the druid who was a large creature, could not reach the rider. Another player responded by quoting the rule that a rider is considered to be in any square that his mount occupies. But for the life of me, I can't see that rule making sense when a mount is two or more sizes larger than the rider, and the attacker isn't within at least one size category of the mount. If the druid was huge (tall), I might have let him have the grapple. Was this a poor rules call on my part?

You did the right thing for the wrong reason.

In this situation, the large druid could reach the rider. The rider's lance gave him 10 ft. reach. The large/tall druid had 10 ft. reach. Your other player was right; a rider is considered to be in any square that his mount occupies. So the rider could reach 10 ft. from the squares he shares with the wyvern. And your druid could reach the rider.

But your druid can't grapple the rider that easily because he can't complete Step 4 of the grapple. Steps 1-3 should have been no trouble, but Step 4 would require the druid to move into the rider's squares, and those squares were off-limits for the druid because he wasn't mounted up on the wyvern. If your druid did succeed at steps 1-3, either he couldn't complete step 4 or, if you did let him complete step 4, he would have ended his movement in an illegal square and would have been bumped back to the most recent squares he occupied.

Second: An arguement broke out that the lance that the rider of the gargantuan wyvern was using was not long enough to be able to reach past the mount's bulk on a charge or in melee.

This one shouldn't be an argument. A reach weapon of the appropriate size doubles your reach. Since you share squares with your mount, you can measure your reach from any corner of the squares you occupy, just like a human on a horse only larger. The downside of using a reach weapon from a mount is that there are many more squares that are considered "adjacent squares" where your reach weapon does not threaten (the "reach donut").

For a medium rider wielding a medium lance on the back of a huge mount, the lance can reach any of the 24 squares that are 10 ft. away (recalling that reach weapons don't follow the 5-10-5 rule, so a reach weapon will threaten the corners, too).

T T T T T T T
T - - - - - T
T - m m m - T
T - m m m - T
T - m m m - T
T - - - - - T
T T T T T T T

Where 'm' shows the mount's squares, shared by the rider, and 'T' shows the squares that the lance threatens and '-' shows the squares that are adjacent to the mount and rider where the lance does not threaten.

 

[The Forsaken One]

Just have the druid snatch away the lance and pole vault up there.

Bye
Thanee

I'd allow that with a readied action

 

[kjenks]

Hes got MONKEY GRIP, you yourself gave it too him. Just let him wield a 2 times oversized lance for 20ft reach. Whats the problem? Just take a -2 on attack thats all theres to it.

An over-sized reach weapon does not grant 20 ft. of reach. There's a FAQ entry on this topic:

FAQ
===
How do reach weapons work if they are of a different
size than the creature wielding them? Say, an ogre wielding
a Small or Medium glaive, or a human with the Monkey
Grip feat wielding a Large ranseur? What is the reach for
each situation?

A reach weapon doubles its wielder’s natural reach, but
only if the weapon is at least of an appropriate size for the
wielder. Wielding a "too-small" reach weapon grants no reach.
An ogre (Large) wielding a Medium or smaller reach
weapon gains no reach from the weapon, and could thus attack
foes either 5 feet or 10 feet distant (as normal for a Large
creature wielding a non-reach weapon).

A human (Medium) wielding a Large or larger reach
weapon could attack a creature 10 feet away (but no further),
and could not use the weapon to attack a creature 5 feet away
(as normal for a Medium creature wielding a reach weapon). A
human wielding a Small reach weapon would gain no reach
from the weapon.

The Player's Handbook isn't as clear on this as it could be,
although an example of reach in action on page 113 in the
Player’s Handbook provides pretty strong support: "A typical
Large character wielding a reach weapon of the appropriate
size can attack a creature 15 or 20 feet away . . ." [italics
added]. While this reference doesn’t mention the ability to
wield a reach weapon larger than the appropriate size, allowing
such a weapon to grant reach to its wielder is a reasonable
extension of the spirit and intent of the rule.
===

And for the rest theres no way in hell that Id let a large person grapple someone on a gargantuan mount. If he wants to know how you hit stuff, just give him an oversized lance like I said and explain to him that a wyvern can tilt/turn while flying in a straight line using flyby ect to give his rider even a better position to strike if needed.

The mount's Flyby attack feat doesn't help the rider attack. There's a FAQ entry on this, too.

FAQ
===
Suppose you have a flying mount: a griffon. What do
you have to do to use the ride-by tactic in the air or in
swooping down on a target? Do you need to take the Flyby
Attack feat? Or does the Ride-By Attack feat cover flying
mounts as well? For the mount to make a single attack
when riding or flying by, does it also need to somehow gain
a feat? If so, which one?

If the example rider wants to make attacks while flying by
foes, she needs the Ride-By Attack feat and needs to charge
(and fly) in a straight line (see the next question). For the
mount to attack in the same situation, it would need the Flyby
Attack feat. The rider's Ride-By Attack feat won’t help the
mount attack, and the mount's Flyby Attack feat does not help
the rider attack.
[...]
===

I added the italics.

 

[kjenks]

I'd allow that with a readied action

The rules don't prevent you from mounting up on a hostile mount.

You can ready a standard action, but this would be an attack action (Disarm) and a move action (jump) followed by another move action (mount up) and another attack action (grapple). Ain't gonna happen.

 

[moritheil]

Also, don't forget that a mount can take an AOO when someone attempts to grapple its rider, since it's also threatening the nearby area.