Negative Levels are a Direct Result of Energy Drain; but not Synonymous.
Enervation vs. Energy Drain
Enervation's Negative Levels don't last long enough for permanent level drain.
Usually, negative levels have a chance of permanently draining the victim’s levels, but the negative levels from enervation don’t last long enough to do so.
Energy Drain's negative levels do last long enough for permanent level drain, and also bestows a lot more per spell cast (metamagic aside); allowing that many more chances of failure to remove negative levels.
There is no saving throw to avoid gaining the negative levels, but 24 hours after gaining them, the subject must make a Fortitude saving throw (DC = energy drain spell’s save DC) for each negative level. If the save succeeds, that negative level is removed. If it fails, the negative level also goes away, but one of the subject’s character levels is permanently drained.
Unholy weapons are imbued with the essence of evil [arguable connected to necromantic] energy and "curse" those that wield it, bestowing a negative level as long as it is wielded, but since it is not a permanent effect, it isn't drained.
It bestows one negative level on any good creature attempting to wield it. The negative level remains as long as the weapon is in hand and disappears when the weapon is no longer wielded. This negative level never results in actual level loss, but it cannot be overcome in any way (including restoration spells) while the weapon is wielded.
This is, essentially, a temporary Clone. Your clone, in this case, is exactly like you, but "treated" as if you had two negative levels.
Treat your duplicate as yourself with two negative levels for the purpose of determining the powers to which the duplicate has access (while the duration of this power lasts, those negative levels cannot be removed by any means).
So in this instance, your clone will have double the following:
- -1 on all skill checks and ability checks.
- -1 on attack rolls and saving throws.
- -5 hit points.
- -1 effective level (whenever the creature’s level is used in a die roll or calculation, reduce it by one for each negative level).
- If the victim casts spells, she loses access to one spell as if she
had cast her highest-level, currently available spell. (If she has
more than one spell at her highest level, she chooses which she
loses.) In addition, when she next prepares spells or regains spell
slots, she gets one less spell slot at her highest spell level.
Now, if your clone dies before you rejoin, you are essentially losing a part of your life force. Necromancy has been described as the essence of life and death, so a part of your essence has just died off, and you would gain a negative level.
If your duplicate dies before the duration expires, no rejoining occurs, and you gain one negative level.
Now, if you die and your essence lives, then a bigger part of your life force has just died off, so you would now be your clone, with the two negative levels.
If you die, your duplicate remains in existence, and is for all intents you, but with two negative levels. (Once the duration expires, one of the negative levels immediately converts to one lost level; the other negative level can be removed by standard means.)
Has it been effectively drained? Arguably, yes, you drain a part of your essence so that part of your essence can have a life of its own. Although the psychic power of Fissure doesn't mention necromancy, in the power description, it can be inferred by the language used.
Context of reality could be like this; your energy gets drained whenever you donate blood (Energy Drain). The nurse literally drains your blood and puts that into a container. That blood, genetically, is you, just a smaller part of your entire blood supply (Fissure). If that blood supply isn't put back into you, you feel drained for probably a day (negative level) until your body naturally replenishes itself (removes its negative level). If that blood supply is put back into you, you feel rejuvenated immediately (negative level is gone).
We can look at Undead for example, of creatures that are immune to Energy Drain.
- Not subject to critical hits, nonlethal damage, ability drain, or
Undead, have no life force. Therefore, there is literally no "energy" to drain. What energy is being drained? One could infer Positive Energy. Undead are the result of reanimation from Negative Energy effects. Negative Levels don't count towards them, because they are made of that Energy.
Also, constructs are devoid of a life force. They are immune to necromantic effects.
- Immunity to poison, sleep effects, paralysis, stunning, disease, death effects, and necromancy effects.
Energy drain is a necromantic effect.
What would happen if a lawful good vampire picks up an Unholy Weapon? That would be a good question in and of itself. Since the Unholy Weapon, technically isn't draining the vampire, I would say both effects happen. He gains a negative level, but is also healed. That would have to be a judgement call by the DM.
Some creatures, such as Hellbred, can use Unholy Weapons, even though they are good aligned. Once again, since it is more of a curse, they aren't being drained, they can pick that bad boy up and swing it around for the cause of good.
Negative Levels, although the result of Energy Drain, are not necessarily Energy Drain in and of itself. Some other times, it can be simply a curse or a consequence of doing/having something that you are not supposed to.
Your DM would have to decide if Negative Level consequences count as necromantic effects. It would make sense, not break anyone's game, and allow that lawful good vampire to wield weapons that Hellbred can.