Limbo

How much control does a person have when controlling matter on Limbo?
Very little, and not for long. Control is little more than selecting and mixing element-dominant traits - you can create a lump of earth surrounded by air, or a ball of fire encased in earth, or possibly even a little island rock in a larger pool of water with a small dome of air above. Fine control and the capacity to "mix" elements are beyond most minds; only the very rare minds known as anarchs have the capacity to effect real control over shapes and structures, and there's no test to be one - you either are (DM's discretion) or you are not.

==== 1Can he shape the terrain however he wishes (i.e., create a field of earth with a 10-ft mound, a 10-ft pit, a raging river, a 5-ft flaming firepit, and strong winds all within the limits of the area)? ====

No.

Is there gravity within the controlled matter?
Limbo's normal gravity rules apply. The individual controlling matter in a given area has no influence on the subjective gravity of others.

==== Can he change the terrain whenever he wishes (i.e., create a small hill underneath himself during combat to give himself higher ground, or create a pit under an enemy to cause him to fall...perhaps making the pit have stone spikes for extra affect, or dismiss air within an area to suffocate enemies)? ====

A non-anarch cannot effect such changes. X-dominant or X-and-Y dominant with very primitive shapes are the extent of control.

==== I just ran a session that takes place in Limbo; first time using the plane. And these were questions that came up during the first combat. My players have a (frustrating) habit of trying to figure out how they can take advantage and "beat the system" at every turn. I should have seen this coming and verified these things long ago. ====

Yeah, don't run combat in Limbo. The whole thing where they fly 10-40 feet in a random direction every round through terrain they can barely see through at the best of times which all shifts randomly in 25-foot radius bubbles every 1-10 minutes from open air to solid rock to pure flame that deals 3-30 damage to those exposed and sets them on fire... even with attune form or avoid planar effects you can still get encased in rock and unable to usefully accomplish anything. The rules for Limbo were decidedly slapdash, more's the pity.

In case they attempt it, they also can't attempt to annihilate structures in Limbo, as they can only control the chaos matter, not anything built with actual materials.

==== What if the person controlling the area was an Anarch (they hired an Anarch as a guide). How would you answer those questions in terms of the controller being an Anarch with minimum Anarch abilities (Wisdom around 20-22)? ====

I'm just trying to get a feel for what kind of control a person would have in Limbo depending on how good he was at controlling.

Anarchs are capable of creating structures and even basic vegetation. The spell perinarch, from the Planar Handbook, gives some examples of introductory complex control, but points out that whatever is accomplished would offer a Reflex save to avoid. Anarchs cannot:

• Create void pockets without air. Something must occupy that space. • Impose gravity upon others. • Create exotic elements (no osmium, no poison). • Create sentient life. • Control specific conditions of shaped area (cannot make it freezing cold, cannot control weather)

Anarchs can:

• Create walls of earth, water or fire (Reflex to avoid) • Create pit traps or pits filled with flame (Reflex negates, although subjective gravity) • Raise or lower terrain

So I would recommend:

• Adding a major feature (wall, pit, difficult terrain) is a standard action. • Adjusting terrain for a tactical advantage should be adjudicated with a small ad-hoc circumstance bonus or penalty.

Odds are that fighting in an anarch's region of control is far less terrible than fighting in raw Limbo, so overall it's a mercy. Remember that any protections against Limbo (attune form, etc) also apply against some forms of anarch-shaped trolling (fire in particular, but anything that emulates elemental dominance applies).