Paul Shave (see previously: Atom Adventure, Pirate Island) went for broke with his last adventure, moving from the Atom to the more capable BBC Micro. Back in 2014 he was contacted by Anthony Hope (one of our regular commenters); Paul helped Anthony beat Xanadu Adventure, and as Paul himself stated in this interview:
I’m pretty sure he [Anthony] was the first.
In other words, at the time of release, it was too difficult for anyone to beat. Will it dethrone Quondam as the most difficult adventure ever?

From Every Game Going.
Having built up that hype, I should add the caveat that “difficulty” is not really a linear spectrum and has lots of elements mashed inside. Judging by Atom Adventure (which Anthony Hope claims is sort of a mini-version of Xanadu) the difficult aspects go in a rather different direction.
Quondam involved paying attention to extreme object micro-interactions, and was tightly packed with nearly every action requiring some sort of puzzle to be solved.
Xanadu’s difficulty is in randomization and optimized timing. Regarding the latter, most games — even the evil Phoenix mainframe ones — gave a lamp with a relatively generous lifespan that doesn’t require watching every step. The Paul Shave games all have, on the other hand, given exactly the amount of light needed, and not a step more; this gets to the level of being cautious what entrance to take into a cave as one entrance uses up a precious extra move of light and will eventually cause failure.
The randomization I’ve seen places some objects at random, so despite the absolute optimization condition above, you still have to deal with improvising a path (and Atom Adventure, at least, occasionally gave a literally impossible layout).
Absolutely tight limits and randomization make for an incredibly high-pressure experience. The closest comparison I can think of is Madness and the Minotaur (which I played last year) but while Madness and the Minotaur arguably had even more randomization, it at least tried to provide ample opportunity to “refresh” decaying health and light sources, going as far as randomly spawning a new refresh after one gets used up. I don’t expect any such niceties here.
As is usual for authors still under the shadow of Crowther/Woods, the objective is to gather treasures. As is slightly unusual, the instructions state you need to DEPOSIT the treasures rather than DROP them to get points. The instructions don’t give how many treasures there are or even a maximum possible score.

From Acorn User, November 1982. Gareth Pitchford has collected other advertisements and reviews as well.
Before embarking further, I should also note this odd portion from the instructions:
There are lots of dwarves and dragons about. To kill them, you need weapons (you can kill them without, but it’s very unlikely). A sword has a weapon count of 10, an axe’s count is 5. To kill a dragon outright, you need a weapon count of 20; for a dwarf it’s 15, but if you throw an axe at a dwarf you always kill it. Your chances of killing monsters are proportional to your weapon count.
It sounds like all the weapons being carried contribute to your weapon “count” (as opposed to just using your best one), so if you have a sword, an axe, and a ??? you can outright kill dragons, but only have a probability of doing it with a sword. This feels weird and uneasy and I suspect there’s a trick hidden here somewhere.

The “1 or 2 Adventurers” question is interesting, but I’m going to ignore that feature for the moment.
You start in an “adventurer shop”, and no, you can’t just buy two swords right away for some dragon hunting action; the shop runs out. I’m unclear what’s optimal here but I’m the “messing about” portion of my gameplay so far so I’m trying everything out, including the postcards.
Speaking of postcards, I did my usual process for ultra-hard games and created a verb list right away. MAIL is not on my usual-test list but I thought it might work on the postcards.
CLIMB, SWIM, READ, BREAK, OPEN, CLOSE, DRINK, EAT, KILL, FILL, LIGHT, THROW, HIT, UNLOCK, LOCK, POUR, PAY, FEED, PULL, SCARE, USE, INSERT, KICK, BUY, STAB, PLAY, FIGHT, SING, CROSS, MEND, MAIL, DEPOSIT
A few to keep in mind as I move forward: MEND is quite out-of-the-ordinary (only previously seen in Hezarin) as well as SCARE (which I’ve seen maybe twice?) I also wouldn’t immediately think to SING anywhere, and USE being in play means I’ll need to test it in lots of places. Some of the typical magic-item manipulations like WAVE and RUB are out of play, but there’s always magic words.
After shopping, you go out to find a locked grate, Adventure style, and no keys; your first treasure, a ruby ring; and an empty bottle.
There’s a mostly unmappable forest (I tried, you can see my attempt above, but items started getting moved around and some exits shift at random); the only purpose of going in is to finding a pagoda.

Thankfully for maze-mapping, the “diagonal” directions of NE/NW/SE/SW are not allowed.
In addition to the outside being a treasure deposit area, you can go IN and then DOWN into darkness for what I assume is a random experience. I only got two moves in before getting wrecked by a dragon.
I’m assuming the dragon’s placement is random, and I’d get something less aggressive on a second playthrough. I’ll have to keep throwing dead bodies at the cave and return with a report.
Maybe you can throw the postcards at a dragon and give it a nasty paper cut. ;)
Lol. Hold that thought. ;)
I am ready to try incredibly random things with the postcards now (although I need to finish the writeup of my last session first).
While cards do nothing on the dragon, this did give me an idea that worked. Next update in a few days.
I think for sheer sustained masochism you still can’ t top Acheton. Incredibly tough set piece puzzles, soft locks a plenty, an extremely tight lamp timer (even tighter than BrandX) it is studded with sudden deaths and it goes on and on and on. The protracted nature of the torture gives it my gold medal vote for harshness.
Acheton is certainly a worthy contender. Any of the first four Phoenix games are, really.
At least Xeno and Fyleet are a way off yet. Oh boy..
I’ve beaten Fyleet before! It was long enough ago that I’ll probably have forgotten the puzzles before I arrive (I did beat it more legit than my first go-round at Hezarin prior to my blog, at least). I might even have scans of my old maps backed up somewhere.
There are a few strange puzzle solutions in Fyleet; well done on completing it. For example why would you take a library sign into a rabbit warren maze? I am just having a go at Quondam for the first time in ages. Gawd it really is a stinking hard game.
Heavens to Murgatroyd I had forgotten how ridiculously hard Quondam was. Aside from the web puzzle (which is at least logical) I have discovered only17 locations in three days’ solid masochism and found ten ways to die, not counting the unwinnable states I have manoeuvred myself into. When saving and examining kill you and even your rucksack is useless you know you are in trouble. And I certainly didn’t expect the Spanish Inquisition neither . Maybe I was wrong and it is harder than Brand X and Acheton.
It really is the only adventure game I can think of where saving your game kills you.
Yes; I can think of a few (all from this stable!) where saving can make a game unwinnable but that really is taking things to extremes. On the plus side I have just worked out the banking system.