I think my idea of a grail doll may differ from other people in that I believe it's quite possible to have more than one grail doll. I do believe it's "that one extra-special doll that you may possibly never obtain". But, on the off-chance you DO obtain it, who says you can't find yourself a new grail doll afterwards?
For example, I used to have a grail doll, Luts Elf Lishe. She was for a character who'd started out as a minifee Lishe, I re-embodied her as an Elfdoll Aurora, but the look never quite fit. I always envisioned her with Lishe's features, only since she's half-elf, she needed pointy ears. Thus, elf Lishe. Anyway, I always saw her pop up on the marketplace, but always at $900 or more, which is more than I'm willing to pay for a secondhand doll. So, I figured I'd never get her. Then, someone who was leaving the hobby sold her 2006 type 2 version for a really cheap price (in comparison with the usual), so I snapped her up. Thus, I obtained my grail doll.
NOW, I have at least two more grail dolls I'd love to get my hands on, but probably won't.
The first is a Dollmore Lusion Dahlia, the elf-eared limited girl. First off, she's HUGE. Not much room to put her at the moment, although I'm hoping that will change in a year or so. Secondly, she's EXPENSIVE. Over $1500 if I recall correctly. Thirdly, she's limited to like 30 or something? Which means by the time I can save up funds, get a bigger place, what-have-you, she'll probably be sold out. Or maybe she won't, I dunno, but right now I'm not holding out hopes of actually obtaining her (for a good long time, at least).
Second doll is Luts Elf Shiwoo. I want him for another character of mine, but he's even harder to find than Elf Lishe, and tends to be that much more expensive. Just his head goes for as much as a full SD. Luts never released a second version of him and the first version doesn't come up for sale very often (and I wasn't even in the hobby yet when he was first released). But I'd love to get my hands on one *for a reasonable price*. Shiwoo is one of my favorite sculpts (thus the four Shiwoos I already own of varying sizes, all but one with elf ears), and this particular elf character of mine would never work with any other sculpt but his. I keep holding out hope that Fairyland will release a new version of elf Shiwoo as one of their limited heads or something, but who knows if/when that will ever happen? Until then, he's definitely on my "grail doll" list.