How exactly do negative levels affect undead?

Question

Im working on a necromancer and have lots of weak skeletons. Ive been researching ways to make what I have better since getting better skeletons is less likely, and Im more likely to get more of these weak ones in the future.

There are several sources of negative levels but there is no single ruling that I have been able to find in how undead are affected by negative levels. The closest thing to a single ruling is Energy Drain and Negative Levels which mentions nothing about the interactions with undead.

I am playing a Spheres of Power caster but their Death Sphere Ghost Strike abilities work the same way as Enervation, but neither mention that the other effects of negative levels are not in effect.

An undead creature struck by the ray gains 1d4 x 5 temporary hit points for 1 hour.

And from SoP

If this ability is used on an undead creature, it instead grants the creature 5 temporary hit points per negative level, which last for 1 hour.

Since undead gain temp HP but no other effect is listed, do the penalities such as the -1 on attack rolls for each level and -5 HP still apply? I would like to think not due to the next which is a weapon called Life Drinker.

This +1 greataxe is favored by undead and constructs, who do not suffer its drawbacks.

This suggests that negative levels while applied to undead do not have their normal penailities, but is vague in if thats a property of the weapon or just some unstated rule, but when take with Enervation that leans more towards a rule.

Now we look at the Holy weapon property.

It bestows one permanent negative level on any evil creature attempting to wield it.

Since undead are typically (or always) evil, does that not mean they can use holy weapons and gain some extra temp HP (this is assuming that the negative levels have no other effects from above)?

Finally, if you were to have a small group of undead with energy drain attacks, such as Wights, could they prepare for combat by attacking each other and gaining high amounts of temp HP?

So to end with, here are the questions clearly:

• What specific effects do undead suffer from negative levels, and does this depend on the source such as from holding a holy weapon?
• Can undead be killed by negative levels?
• Is there no limit to the amount of negative level temp HP they can have if not killed by negative levels?

Answer 1

Undead creatures are immune to energy drain

Energy Drain is listed as one of undead type immunities:

Not subject to nonlethal damage, ability drain, or energy drain. Immune to damage to its physical ability scores (Constitution, Dexterity, and Strength), as well as to exhaustion and fatigue effects.

As such, any effect that is described as Energy Drain should not affect undead.

Naturally, (most) effects that cause energy drain, or even negative levels, require Fortitude saves, which undead are also immune:

Immunity to any effect that requires a Fortitude save (unless the effect also works on objects or is harmless).

Similarly, constructs are also immune to effects that require Fortitude saves, so they are safe from both energy drain and (most) negative levels because of that.

But not to negative levels

While energy drain may cause negative levels, negative levels are merely a consequence of energy drain, but not necessarily the same thing.

Some spells and a number of undead creatures have the ability to drain away life and energy; this dreadful attack results in “negative levels.”

As you pointed out, there are other ways to gain negative levels other than having your energy drained. Depending on the effect, it may not care if the creature is undead or not, as undead are not naturally immune to negative levels.

The metamagic feat, Thanatopic Spell, is known for being a source of bestowing negative levels to undead.

Undead are susceptible to spells augmented by this feat, as it retunes the negative energy to be harmful to them. A thanatopic spell that would kill a living creature (such as by giving it negative levels equal to its Hit Dice) destroys an undead (though undead such as ghosts, liches, and vampires may reform as normal). Undead affected by thanatopic spells that give negative levels automatically make their saving throws to remove negative levels after 24 hours.

But this is the only effect I know that can allow an undead creature to obtain negative levels. Note that even this feat suggests that undead are immune to negative levels, as those nearly always require a Fortitude save:

Normal: Defenses such as death ward negate death effects, negative levels, and energy drain. Undead are immune to these attacks.

As for the effects on undead, when applicable, are the same:

For each negative level a creature has, it takes a cumulative –1 penalty on all ability checks, attack rolls, combat maneuver checks, Combat Maneuver Defense, saving throws, and skill checks.