How does the oracle life revelation ability Energy body effect dhampires, can you heal them with it? Since the ability technically makes you a positive energy elemental...
Damphir are treated as an undead by Energy Body
When an oracle of life becomes pure life energy, they are like a spirit full of positive energy while the effect lasts, and cause damage on undead creatures that attack them and can damage undead creatures by touching them.
Energy Body (Su): As a standard action, you can transform your body into pure life energy, resembling a golden-white fire elemental. In this form, you gain the elemental subtype and give off a warm, welcoming light that increases the light level within 10 feet by one step, up to normal light. Any undead creature striking you with its body or a handheld weapon deals normal damage, but at the same time the attacker takes 1d6 points of positive energy damage + 1 point per oracle level. [...] Once per round, if you pass through a living allied creature’s square or the ally passes through your square, it heals 1d6 hit points + 1 per oracle level.
Which means the damphir are damaged by this if they attack your character without a reach or ranged weapon, as they are considered undead for effects of positive and negative energy:
Negative Energy Affinity: Though a living creature, a dhampir reacts to positive and negative energy as if it were undead—positive energy harms it, while negative energy heals it.
The damphir are treated as undead creatures for any effects that makes a distinction between undead and non-undead, even though they are also considered living creatures. The healing effect does not say it is a positive energy effect, but that is implied on the ability.
But you can choose not to affect the damphir if either walks through each other walk through your space:
You may use this ability to heal yourself as a move action. You choose whether or not to heal a creature when it passes through your space. You may return to your normal form as a free action. You may remain in energy body form for a number of rounds per day equal to your oracle level.
Do note, however, that if you move through the damphir, you cannot choose not to heal them.
Energy Body is a Positive Energy Effect
A topic that is often debated is wether a healing effect uses positive energy or not. For instance, the paladin's Lay on Hand ability lacks any wording saying it is a positive energy effect (though this FAQ Item fixes that). Usually, people assume that if an effect does not state the correct damage type, it is untyped (and they are correct), but this leads to many issues when talking about positive/negative energy and living/undead creatures.
While Energy Body does not state it is a positive energy effect, this FAQ Item covers that problem, and properly classifies the effect properly:
Energy Body is a positive energy effect because it heals living creatures.
Positive and Negative Energy: These two terms show up in a variety of abilities, but they have no definition outside those abilities, and the abilities aren’t always consistent. How do positive and negative energy work?
Positive and negative energy are two damage types, though despite their name, they are usually not included on the list of energy types you can choose with spells like resist energy or feats like Elemental Spell. You’ll sometimes come across both the phrasing “deals X damage; this is a negative energy effect” and the phrasing “deals X negative energy damage”; these two are functionally equivalent.
Positive energy often heals living creatures, though not always (for instance channeled positive energy to harm undead or the life blast spell). It often harms undead creatures, though not always (for instance channeled positive energy to heal living creatures). Individual effects will tell you whether they heal living (if they mention healing without specifying what they heal, they always mean only living creatures), harm undead, or both. Positive energy never heals or harms creatures or objects that are neither living nor undead (such as constructs), and it never directly damages the living or heals undead, barring some special effect that explicitly changes this like a dhampir’s negative energy affinity. These rules extend to the fast healing from positive-energy attuned planes as well (though overhealing on a major positive-energy attuned plane can be dangerous as well); only living creatures gain fast healing on such a plane.
Negative energy works just as described above for positive energy, reversing living creatures and undead in all cases (it often heals undead, it often harms living creatures, if it mentions damage without specifying what it damages, it always means only living creatures, and so on).
Also, though not properly addressed on the undead creature type, they did include a rule about how undead creatures behave against positive energy on the Advanced Race Guide (they are hurt by positive energy).
If the energy type of Energy Body is still unclear, remember that the only source of life energy on the rules is the Positive Energy Plane.
Actually the rules about how undeads react to positive energy are vague enough that you need to have a specific line on an effect for it to react differently on undead and living persons. For example the cure spells all have this line:
Since undead are powered by negative energy, this spell deals damage to them instead of curing their wounds.
(note that this has been errated at some point, the previous version was way more confusing)
Besides these lines, if a positive or negative energy effect doesn't specifically indicates that it makes the difference between undeads and livings, it has no reason to act differently. As the Oracle's mystery doesn't specify anything it can heal undeads (and damphirs).
However it seems like the devs themselves consider that a positive energy healing should by default hurt undeads:
As a result of a successful knowledge check, a PC can learn that: "Undead are healed by negative energy and harmed by positive energy".
Many monsters (including damphirs) have the Negative Energy Affinity ability:
Though a living creature, a dhampir reacts to positive and negative energy as if it were undead—positive energy harms it, while negative energy heals it.
This is contradictory with the rules, but can be understood as "if a positive or negative energy effect that behave differently for undeads or livings should affect a damphir, consider them as an undead".
Alternatively you can choose to houserule this and consider that by default positive energy healing hurts undead. This makes sense in the case of this oracle's mystery.