Does a template that grants HD (lycanthropy, but also multi-headed, zombie, etc.) necessarily give XP to be at that new character level? Is there some requirement somewhere that one has to have enough XP for their HD?
If Alice, a second level human fighter with 1001 XP, contracts werewolfery, she gains 2 animal RHD. She's a Fighter 2/Animal 2. Does she have 1001 XP, or did she jump up to 6000 XP (normal for a character with 4 HD)? As an afflicted werewolf, she has +2 LA, too—does that affect things? Normally gaining LA doesn't affect one's XP.
Also salient is page 58 of the PHB, which reads:
A character can advance only one level at a time. If, for some extraordinary reason, a character's XP reward from a single adventure would be enough to advance two or more levels at once, he or she instead advances one level and gains just enough xp to be 1 xp short of the next level. Any excess experience points are not retained.
- As a 6th level character (Fighter 2/Animal 2/+2 LA), Alice would normally need 21,000 XP to level up. Also normally, she'd need XP equal to 1000 × her ECL, or 6,000. Before she was bitten, she had 1,001 XP. How much XP does she have, and how much does she need to reach Fighter 3?
- Werewolf Alice gets in a fight with a vampire, as is natural for a werewolf. She gets a negative level in the process. What happens to her XP when she fails her save and loses a level? Normally, that should set her XP to the halfway point of her new level, Fighter 2/Animal 1/+2 LA, so between ECL 4 and 5, or 12,500 XP.
- Say Alice starts adventuring and reaches 21,000 XP (however much that took). What happens when Alice receives a remove curse spell during the full moon? She loses her lycanthropy, and with it her animal RHD. What does that do to her XP total?
- If her XP total is unchanged after being cured, does Alice immediately level up? How many times?
- Can a werewolf cleric just level people up by giving them lycanthropy then curing it? If that doesn't work, what about mixing in level loss to set XP to the halfway point between two (higher than pre-lycanthropy) levels?
Best Answer
Savage Species on Effects of Transformation, in part, says, "Once a character transforms, she has a new effective character level. This ECL determines how many experience points she needs to gain her next class level…" (145). Then it offers an example:
Realize that Savage Species was conceived as a standalone project then more or less shoehorned into the mainstream Dungeons & Dragons product line (more here), so its rules must be examined with some caution, but absent anything else in this regard, the above seems, to a point,—and let's be generous—awkwardly playable.
So with this in mind, it appears that, by extension, there's no change to a creature's XP upon a creature somehow acquiring Level Adjustment or Hit Dice during play, no matter how that occurs. For ease of reference, the difference between a creature's current XP and the creature's XP total needed to reach its next ECL is hereafter called the creature's XP debt.
Questions & Answers
I have rephrased these for technical accuracy. I hope that I've maintained the spirit of the original questions.
When a creature during play adds 1 or more Hit Dice (HD) and/or sees its Level Adjustment (LA) increase by +1 or more (like from acquiring a template), does the creature receive experience points (XP) as if it had earned sufficient XP to gain character levels to match the creature's new Effective Character Level (ECL)?
No. The creature's XP total is unchanged. The creature simply must earn more XP to reach its next level. For example, Alice the human Ftr2 possesses 2,999 XP and is bitten by a werewolf. She gains 2 animal HD and +2 Level Adjustment (LA)—so that her Effective Character Level (ECL) is 6—, but she still possesses only 2,999 XP. She must earn a total of 21,000 XP (i.e. enough for character level 7) to gain her next experience level and become, for example, a werewolf human Ftr3.
Is the game constantly checking if a creature possesses sufficient XP for its HD or LA?
It appears not. If a creature is somehow capable of gaining HD asymmetrically nigh-infinitely—maybe involving the 4th-level Drd spell awaken [trans] (PH 202) with shenanigans—, the creature's ECL increases, yet its XP total is unchanged.
What if a creature suffers level loss before it's paid its XP debt?
Pretty much the same thing that happens to everyone: "the creature’s level is…reduced by one" (Monster Manual 308) and the creature's "experience point total is immediately set to the midpoint of the previous level" (Dungeon Master's Guide 296). While it seems reasonable to lose the most recently gained "level" first, that may end up being a lycanthrope's animal HD, and that may not sit well with the GM. No matter what, though, the character's XP total will be reduced by ECL 1 (e.g. human were-wolf Ftr 2 Alice loses 2 levels to a vampire, the first lost level dropping her from 2,999 XP to halfway between 1 and 2, or 500 XP, and the second lost level dropping her to halfway between a handful of thrown d4s and a flipped table. I mean, the game doesn't say, okay?)
What if a creature loses HD or LA after it's paid its XP debt?
The game doesn't address this. That is, Alice earns that 21,000 XP, becomes a Ftr3, then is cured of lycanthropy. The DM and Alice should determine a reasonable compromise. The DM may just let Alice catch up to her XP total by letting her take right now a couple more levels of rogue or whatever, and that should be playable enough for the (presumably?) extremely rare occasions that this would occur. Gaming this is possible but weird. For example, a level 1 human wizard that's affected by an enlarge person spell contracts lycanthropy after having been bit by a were-tyrannosaurus hill giant. The Wiz1 spends the next equivalent 20 levels adventuring until he's a level 2 wizard, gets cured of lycanthropy, and is suddenly an epic wizard. Really, though, that's a plot not a PC path.
Can a creature gain XP by gaining then losing HD or LA?
Not unless the template itself has such an effect, and I don't know of any that do. Since a creature's XP total remains unchanged by the application of the HD or LA, if the HD or LA is later lost, there's typically either no effect (if no XP was gained between the application and the loss) or see the previous question, above.
Keep in mind that tinkering with XP totals and level loss is controversial and isn't well-documented by the game (q.v. this question and this question). At some point, the DM is going to have to be involved. After all, you typically can't award yourself XP.