[Build Advice] One of the PCs is secretly a powerful archfey

[Seneschal]

Originally Posted by Pursuivant

For combat durability, also consider extra HP, Combat Reflexes [15 points], Damage Resistance [5/lvl], High Pain Threshold [10 points], and levels of Hard to Subdue [2/lvl].

For non-combat durability consider Resistant (Disease/Poison/etc.)

Good basic HT (12+) is also a decent way of modeling a durable character.

All these traits, especially Unkillable, might be countered by disads like Revulsion, Vulnerability, or Weakness to things like church bells, cold iron, silver, etc., but those sorts of disadvantages can get dangerous really fast.

I've begun assembling the character in GCS and it's, almost to the letter, everything you've listed. I'm actually quite surprised at how (relatively) economical it is. I've opted for iron as a weakness (but not steel, only pure iron - fire pokers, horseshoes, nails, musket barrels...) for its "magical insulator"-qualities. The final write-up looks something like this:

Damage Resistance 1 (Tough Skin, -40%) [3]
Hard to Kill 2 [4]
Hard to Subdue 2 [4]
High Pain Threshold [10]
Regeneration (Slow; Magical, -10%) [9]
Regrowth (Bane (Iron, occasional), -30%; Magical, -10) [24]
Resistant (Metabolic Hazards, +3) [10]
Revulsion (Iron, occasional) [-5]
Unaging [15]
Unkillable 1 (Achilles' Heel (Iron, occasional), -30%) [35]
Unusual Biochemistry [-5]
Vulnerability (Iron, occasional; ×2 injury) [-20]

A mere 84 points to be the fantasy equivalent of Jason Voorhees, shrugging off whatever you pump into them and getting up eventually no matter what's sticking out of them... unless your enemy brings an iron weapon. Then you're at -5 to everything if you get hit, and you treat killing blows from it as if you were a normal human. This is an acceptable dichotomy that comes with playing a "monster character", and the danger it presents is a good motivator to keep your inhuman nature under wraps.

I like that none of the abilities are absolute - you can still try to blow the faerie lord up with gunpowder, or put them to sleep by dipping them in a vat of opium, but most solutions are unreliable unless you know their weakness. Also, it allows the character to have fairly modest attributes, and thus play on a similar level as the rest of the party.

Originally Posted by Pursuivant

So, serious "sociopathic superficial charisma."

In addition to previous suggestions: Attractive or better Appearance [4+ points]; Charisma [5 points/lvl], Talent (Allure [5 points/level] or Smooth Operator Talent [15 points/lvl]), and Voice [10 points].

All suit the "only pretty on the outside" sort of noble faerie.

For creepy and offputting behavior, in addition to the suggestions that others have made: Bad Temper, Bully, Callous, On the Edge, bad Reputation, Sadism, Supernatural Feature, Trickster, or Unnatural Feature.

Bad Temper seems especially appropriate for the genre.

Possibly any of these traits could have a house-rules Accessibility limitation "Not immediately obvious" or "Not well known" worth at least -40% which limits negative reactions to just people who spend significant time in the character's company.

Keep in mind, however, that many traits which carry reaction modifiers conflict with the sort of social manipulation skills required for a bard-like character.

Additional Suggestions: Clueless, Code of Honor, Delusion ("Human society is like Fae society"), Hidebound, Oblivious.

Absent-Minded and Short Attention Span might be appropriate if the character is particularly flighty. Odious Personal Habit (Capricious) [-15 points] applies if the character has limited control over their moods.

I agree that you should choose which disadvantages are most important and choose the rest as "Controllable Disadvantage" or "Likes and Dislikes" Quirks. E.g., "Fascinated by mortals," "Capricious," "Never forgets an insult or a favor," "Views human society from a faerie point of view."

This is a very comprehensive list, thank you. I particularly like Clueless, which I hadn't considered previously - it fits the idea of an immortal trickster who doesn't really comprehend the passage of time, and thinks that the TL2 tribes he toyed with a few hundred years ago still function the same at TL4.

Given the sheer number of disadvantages, all appropriate, I'll make a list and consult the player on what they'd most be interested in roleplaying. We'll probably stick to one or two major ones; the rest, as you point out, are best left at quirk-level.

Originally Posted by Pursuivant

This is a massive list of powerful advantages, especially the plane-shifting and wish granting bits.

If your preference is a low-powered character, it might be fairer to the player to start play with most of these traits as "potential advantages" which can be accessed as the character develops. Especially for a 5e player, the concept of "leveling up" should be simple to understand.

Amnesia or Partial Amnesia is an expensive disadvantage would could easily explain the character's lack of full access to their powers and would a good way to reduce the point cost of a low-point character.

Some form of Racial Memory could explain access to ancient lore. Blessed, Destiny, or Unusual Background might also justify very limited wish-granting power. Detect might be useful for detecting caches of valuable goods, with the quirk that the character can't benefit from such riches themself.

Everything else depends on how magic works in your game and how much "new system crunch" your player is willing to put up with.

In particular, any system of GURPS Magic will work very differently from AD&D magic, so the player will have to know what they're getting themselves into.

IQ 13+, Magery 2-3, and 15-20 points in spells is a relatively cheap method of creating a character with reasonable "buff"/"debuff" powers. Another possibility might be Modular Abilities and variants on the Affliction advantage.

For 500 points, you could build an incredibly formidable GURPS Magic archmage character capable of casting powerful spells like Enslave, Planeshift Other, or even Wish using the standard GURPS Magic system.

For limited relatively inexpensive "enchant" powers, Imbuement powers might work best.

Jumper (World) is a very expensive advantage which could possibly be reined in using Accessibility (Only from certain places and at certain times of the year) and Limited Uses, Requires Preparation, and Takes Extra Time to bring the cost down to a more reasonable level. This power isn't necessary for an archmage with the prerequisites and power to cast spells like Gate or Planeshift Others.

Illusion effects can be handled using the Create or Mind Control advantages with the Magic power limitation. GURPS Psionics has a list of illusion powers which could easily be adapted to have a magical basis. Standard GURPS Magic also has an extensive list of Illusion and Creation spells.

"Bless," "Curse," or Probability Alteration powers could be magic spells, or variants on Affliction (Attribute Reduction, Bestow Dis/advantage, Negate Advantage, Irritating or Incapacitating Condition). Take a look at GURPS Powers and the Dungeon Fantasy series for inspiration.

For background material, GURPS 3E Faeries has lots of good "fluff" and some templates which could be updated to 4E.

This is what I've been struggling with the most. While the aim wasn't to make a low-powered character per se, I did want to see how few points I could spend (or how heavily I could limit powers) and still end up with an otherworldly entity with a suite of unusual abilities. I've found that Illusion, built correctly and used imaginatively, could stand in for a variety of fey powers, and still be 30-40 points; Snatcher with Recall, Fickle and Unpredictable is a cheap way to represent access to a pot of gold, a dragon's hoard, or a mansion in the faerie realm, complete with a grumpy goblin majordomo that appears to take your order...

However, I'm currently leaning towards using Ritual Path Magic. We've used similar systems in the past, the player had expressed interest in flexible magic, and RPM is the kind of system where I can take a fair amount of cognitive load on myself and just go, "Sure, that could work, it would be a 30 energy ritual."

Plus, it can represent the full scope of fey powers with even a basic point investment. Sure, you can't do crazy high-fantasy stuff without investing a ton, but the starter package is probably the most flexible a GURPS mage can get on a budget. The downside—low-powered RPM magic having to be subtle and hush-hush—actually strikes me as a feature for a mystery/horror campaign.