How does ability damage interact with the Possession spell?

Question

Let's say that a wizard casts possession on a bugbear. Now, this bugbear has spent some time in questionable places and has contracted both Filth Fever and Mindfire, the first being a disease causing Dex and Con damage, the latter being a disease causing Int damage.

The wizard is still possessing the bugbear when the diseases have their next ability damage 'tick'. Who gets the ability damage: the bugbear or the wizard?

The second paragraph of the possession spell description reads as follows (Emphasis mine):

If you are successful, your life force occupies the host body. The host’s soul is imprisoned with you, but can still use its own senses (though it can’t assert any influence or use even purely mental abilities). You can communicate telepathically with the host as if you shared a common language, but only with your consent. You keep your Intelligence, Wisdom, Charisma, level, class, base attack bonus, base save bonuses, alignment, and mental abilities. The body retains its Strength, Dexterity, Constitution, hit points, natural abilities, and automatic abilities. A body with extra limbs doesn’t allow you to make more attacks (or more advantageous two-weapon attacks) than normal. You can’t activate the body’s extraordinary or supernatural abilities, nor can you cast any of its spells or spell-like abilities.

Because of this, I assume that only the body (the bugbear) would get the Dex and Con damage from Filth Fever, though I can't find any rulings on this. But what about the Int damage from Mindfire?

Who takes the Int damage? The wizard, the bugbear or maybe even both?

I find this a difficult question because there are two minds in the same body, unlike similar possession spells like magic jar which removes the original soul from the body.

On a related note (and if this should be a separate question, I apologize):
How would spells like feeblemind work when targeting a body inhabited by more than one mind?

Answer 1

Physical effects will affect the host's body

There are no actual published ruling or developer clarification on this, but it's safe to assume that most GMs will rule it similarly. And the reason I believe there is no developer clarification is that I don't think we need clarification on something that most people agree on.

Most poisons and diseases affect a creature's body, not their mind. So, if you are possessing a creature and they are afflicted with any disease or poison, that will affect you while you are in the host, but will not affect you when you get out.

When possessing a creature, the caster (possessor), is currently using the creature's physical ability scores and not their own, so any damage or drain caused should affect you while you are using the host, and reduce any checks that depend on that ability. If the damage/drain is caused on strength, dexterity or constitution, it will affect whoever is in control of the body.

Mental effects will affect either creature

However, if the poison or disease causes mental ability damage or drain, I see no reason why it shouldn't be able to also affect the possessor, as he is currently inhabiting the target's host and whatever harmful effect is affecting the creature's senses or brain, should be able to affect the possessor, as he is not granted any kind of immunity to that.

For instance, if a poison causes you to go unconscious (like Blue Whinnis), there is no reason to believe that you suddenly became immune to that condition.

Finally, Feeblemind is a compulsion effect.

Unlike other effects (Magic Jar), the host's soul is not ejected from her body when using Possession, and mental effects (like charms) are clarified in Occult Adventures:

Compulsion and charm effects move with the mind or soul of the creature originally targeted. For example, if you cast mind swap on a dominated creature, the domination effect remains active but transfers with the originally dominated mind to its new host body, and a dominated creature capable of possession could possess another creature while still remaining dominated.

If the host’s mind or soul is not displaced from the body, a dominating creature can still telepathically interact with its now-possessed target, even though the target is helpless within its own hijacked body. The dominating creature can even command the target to explain what the host body is doing, if the host has access to its senses.

When both the host creature and a possessing creature occupy a body at the same time, a charm or compulsion effect can target either one.

It is worth reading all that page for clarifications about how Possession and similar effects work, by the way.

As GM, if the player, or an NPC/monster targeting a PC, decides to cast Feeblemind on the "bugbear in front of me", the magic will target the creature that is the owner of the body, unless the caster has some way to know that two creatures are inside that body and decides to specifically call out which one they want to affect with the spell.

This was called out in the Haunted Heroes Handbook, and makes a lot of sense in that context, as you are dealing with ghost or demonic possession, and the very first thing you have to identify, is that the creature is actually possessed by another entity.

If both the host creature and a possessing creature occupy the same body at the same time, charms and compulsions automatically target the host creature’s mind unless the caster is aware the creature is being possessed (letting the caster choose to target either the host or the possessor).

As for any other mental effect, like the Blue Whinnis poison or ability damage/drain to mental scores, I would apply it to both creatures, as they are both conscious inside the body and should be affected equally by anything that hurts their minds.

Answer 2

Both affect only the target creature

Those diseases affect the body of a creature, not their soul. That is why it is a Fortitude save. So only the creature being possessed will be affected.

Feeblemind targets a creature, which in this case will be the creature that is possessed.