I'm a Half-Undead Alchemist thinking of using the Life-Drinker when I mutate into a melee beast.
"Half-undead creatures are harmed by positive energy
and healed by negative energy." (ARG p216)
and
"Each time a life-drinker deals damage to a foe, it also
bestows one negative level on the wielder. Any negative
levels gained by the wielder in this fashion lasts for 1 hour." (UE p.157)
Does that mean I'll be immune to the effects in the first place, deal with the negative levels anyway or will I instead be healed since to create the Life-Drinker you need to know the spell Enervation which states:
"An undead creature struck ... gains 1d4 x 5 temporary hit points for 1 hour." (Core p.277)
My DM is looking it over but I'd like your input as well.
THANKS!
Negative levels and negative energy are two different things. Being immune or healed by negative energy does not confer immunity to negative levels. You'll notice that half-undead have a specific entry for how they handle negative levels:
Half-undead take no penalties from energy-draining effects, though they can still be killed if they accrue more negative levels than they have Hit Dice. After 24 hours, any negative levels they've gained are removed without any additional saving throws.
If you want to get really technical, undead actually aren't inherently immune to negative levels:
Undead are not subject to nonlethal damage, ability drain, or energy drain, and are immune to damage to physical ability scores (Constitution, Dexterity, and Strength), as well as to exhaustion and fatigue effects.
Some spells and a number of undead creatures have the ability to drain away life and energy; this dreadful attack results in “negative levels.” These cause a character to take a number of penalties.
So undead traits stipulate that undead are immune to energy drain, but say nothing about negative levels. The rules for energy drain state that all energy drain effects cause negative levels (so undead are definitely immune to those), but they do not say that all negative levels are energy drain effects. In other words, any effect that causes negative levels that is not explicitly an energy drain effect will actually affect undead creatures.
Now, I'm sure your gut response is "that's obviously not RAI", but without this ruling undead receive no penalty for wielding holy weapons! In fact, every single spell in the CRB that inflicts negative levels explicitly states that undead are immune. In that context, this loophole actually looks intentional, and later authors were simply unaware and failed to include the expected "undead are immune to this effect" clause. It's an interesting edge-case in the rules.
Half-undead take no penalties from energy-draining effects, though they can still be killed if they accrue more negative levels than they have Hit Dice. After 24 hours, any negative levels they've gained are removed without any additional saving throws.
Yes, but the Life-Drinker states that it's "favored by undead and constructs, who do not suffer its drawbacks." (UE. 157)
and"Any negative levels gained by the wielder in this fashion lasts for 1 hour."
So that's even better than normal for undead as you list here:
So undead traits stipulate that undead are immune to energy drain, but say nothing about negative levels. The rules for energy drain state that all energy drain effects cause negative levels (so undead are definitely immune to those), but they do not say that all negative levels are energy drain effects. In other words, any effect that causes negative levels that is not explicitly an energy drain effect will actually affect undead creatures.
It would make sense to draw an equality sign between negative levels and energy drain just to simplify and cut some heavy lawyerism on that part.
Still the main part for your case is the part about half-undeads that Dasrak quoted - you don't suffer the drawbacks in the for of penalties from negative levels. But they can still end you if you go over the line. Negative levels are nothing more than another death counter for you. But that counter can still go up and down.