The fifth Dragon Warriors book, The Power of Darkness was written by Oliver Johnson, who also wrote the third, The Elven Crystals. Apparently he also came up with the name "Dragon Warriors", Dave Morris wanting to use "Dead Men and Heroes", but later agreed that that name didn't say "fantasy" or "buy this product" the way something with the word "Dragon" in it does.
As an aside, what's with that cover? The first introduced Knights and Barbarians as adventures and has a picture on the cover of a Knight and a Barbarian in an adventure, the second was about magic stuff and features a magician on the cover, the third involves (in part) fighting dark riders from a castle and has a picture of an adventuring party fighting a dark rider near a castle, the fourth introduces Assassins and new monsters and the cover has some monsters in it (but I don't think she's supposed to be an Assassin). This book has a Knight on horseback ride up to a ledge over a fiery chasm to face a wizard who is standing on the hand of a giant monster, or maybe a statue of one.
Elementalists
Not Chapter One: Elementalist, and not Chapter One: The Masters of the Elements or something, this book is set out a teensy bit different from the Morris ones. Anyway, while I mentioned in my review of the previous book, Out of the Shadows, that Assassins seem over-powered, I'd give a funny look to anyone who said the same of Elementalists. Or who said that they were as good as Sorcerers. If they said they were better than random humans you met at an inn, I'm not saying I'd totally disagree, but I might have to think about it. A normal human that follows you for 6 underworld adventures gets promoted to a Rank 1 Knight, and an Elementalist that does the same stays an Elementalist, albeit they'd probably be Rank 2 or 3.
At first glance, the Elementalist looks a lot like the Sorcerer (which makes judging their relative worth a lot easier, comparing apples to apples rather than aardvarks). They have the same requirements for Intelligence and Psychic Talent, get the same beginning stats (the book stats these increase with Rank the same as the Sorcerer), they get Magic Points scores. That's scores, plural, you pick one of the four classic elements as your main one and get 3 MPs for that, and the two others that aren't opposed to your main (fire being opposed to water, and earth being opposed to air) get 1 MP each. So, while a Sorcerer starts with 4 MPs, an Elementalist starts with a total of 5. A 1st Rank Fire Elementalist, for example, can cast their Fire spell 3 times, their Earth spell once and their Air spell once before having to recharge. That's spell, singular. For each element, there is one, and only one, spell per Rank. That's not very good.
A 1st Rank Sorceror can cast their only fire based spell, Dragonbreath, up to 4 times per day (or, you know, any 4 of their 6 spells in whatever combination they want), whereas the 1st Rank Fire Elementalist can only cast their only fire spell 3 times before recharging. Dragonbreath is an attack spell that lets Sorcerers shoot fire at their enemies, doing a not inconsiderable amount of damage if it hits. The 1st Rank Fire Elementalist spell is called "Candle". It creates a magic candle only they can see. This is not a useless spell to have, but if you could pick either Dragonbreath or Candle as the one spell you got to cast, you'd almost certainly pick Dragonbreath. Sorcerers don't have to pick one, of course, they get six. The 1st Rank Water Elementalist spell "Rain", creates light rainfall in a 5m radius. The Air Elementalist is creating a gentle breeze, and the Earth Elementalist is producing one piece of fruit per round until the spell expires. The other adventurers are less than impressed. The don't get any offensive spells until Rank 3, the Fire Elementalist can cast a spell on an enchanted arrow and turns it into an Attack 20, (d10, 10) thing briefly. Which is not too bad if you have a bow and an enchanted arrow, I guess. Earth and Air Elementalists don't get offensive (by which I mean "spells to cast at an enemy", not even limiting it to those that'd directly hurt something) spells until Rank 4, though if you'd majored in either of those you'd minor in Fire and Water, so at Rank 3 you'd be able to cast you offensive spell 3 times before recharging for Fire or Water, and your two offensive spells once each for Air and Earth. This is also not very good.
Now, in fairness, there is some decent stuff once you hit higher Ranks. The Rank 8 Earth Elementalist spell, "Give up the Dead" gives you some Skeletons. Oddly, you don't cast it at dead people like you'd expect, you cast it at the ground and hope someone is buried there. 95% chance of 1-12 Skeletons in a burial ground, 10% chance of 1-2 in open ground. Odd way of doing it. Finally you can stop the Sorcerer from making fun of you, they've been raising 1-6 Skeletons with one of the six Rank 5 spells for ages. And will be able to raise a permanent one (or rather, one at a time) when they hit Rank 9 (though, with 1 less Attack than normal, but I think that's a typo). The Rank 10 spells are legitimately powerful, though, the Water Elementalist creating a (narrow) tidal wave that goes half a mile inland, good for fire and forget removing entire villages of annoying NPCs, the others summoning a rather nasty monster. The Air Elementalist monster goes away after it kills person, though, the other ones last for an hour, again good for mass NPC removal. Of course, you don't want to bump into a Rank 10 Sorcerer in a dark alleyway either.
There's a couple of other problems with Elementalists. To regain MPs, Sorcerers just have to wait til midnight and they immediately go back to maximum again. Elementalists recharge by performing a Ritual of Recovery, one for each category of magic they have. They have to have used all their MPs in that category first, which doesn't really matter, but makes me think there'd be a lot of light rainfall and fruit suddenly appear to empty their batteries before recharging. The rituals have to be done at specific times of day, and you can't do more than one in a 24 hour period. So, if you are an Air Elementalist and you've created lots of light breezes, you'd do your Ritual of Recovery at noon. You won't be able to do another until noon the next day, at which time it's too late to do your Ritual of Recovery for Fire, because that has to be done at sunrise. Wait another day for that. Doesn't say how long the rituals take, and it would really be annoying to be in the middle of something and miss your opportunity. The real adventures are off having real adventures while you are trying to get the ability to make candles back. This is also, also not very good.
And another thing. To be able to cast a spell normally, you have to have some special equipment specific to your element. I think you just need the one for your main element and it works for the minor ones as well, it's a bit unclear. If you don't have it, your spells cost twice as much. So your Rank 1 Fire Elementalist can make one magic candle before recharging at sunrise, and that's it. It doesn't give the availability for this equipment like it does for the normal stuff in the first book, nor does it say which count as items for encumbrance. It does give the prices. 5 florins for the stuff for the Earth or Fire Elementalists, 1 GP (10 Florins) for Water, 10 GPs (100 Florins) for Air. That's not very fair, and Elementalists start with only 2-20 Florins. This is also, also, also not very good.
Oh, and in the review of the last book, I mentioned being underwhelmed by the special abilities you get when you reach high ranks. None of that here, Elementalists don't get any of that, no special abilities at all. Maybe Johnson didn't know about what Morris put in the previous book?
On the plus side, Sorcerers start off with a Shortsword or a Staff (and will pick the Shortsword), whereas Elementalists get both a Sword and a Staff. And a Shield and Hard Leather Armour. Good thing, as they'll want to fight in melee combat, not because they are good at it (average human combat stats), but because that's all they can do. So, that's a plus, just a really, really small one. Especially as the Sorcerer starts off with two magic potions. And also a Dagger, which I always thought would be useful to throw at people if they wanted to stay out of a melee whilst contributing somewhat to a fight when they've got no magic to cast. Elementalists not having any useful magic to cast, a Dagger in their starting equipment may have been very useful.
So, that's either "awful" or "a characterful NPC profession" depending on how charitable you feel. I've have no idea how Johnson didn't see the problems there. Ok, maybe you might overlook the problems with the Ritual of Recovery and the special equipment you need if you weren't paying too much attention, but surely you'd notice the 1 spell per Rank thing and the spells being weak? Would require quite a bit of mucking about to get things balanced, perhaps there was a deadline he was struggling with, apparently they had problems with that sort of thing. Myself, I'd get rid of the Ritual of Recovery stuff, you want all your MPs back the same as Sorcerers get, and you want your working day to be the same as others in the party. Probably get rid of the special equipment stuff, looks sorta cool, just doesn't work. You could make it part of the starting equipment and have nobody care about it, or maybe something you need to get at some point but doesn't matter, say, before Rank 3. You could lose the separate amounts of MPs and have spells from your main element cost half as much. You could have more than 1 spell per element per rank. But, just 1 per element per Rank comes out as 50, and doubling that makes 100 spells and only 2 per rank, a lot of effort to create and still not that many options in the way things are set up.
You will note that I said 50, not 40 there, because in any depiction of the four classical elements written after Zoroaster you have to have more than four. Aristole had aether (and thus he's to blame for GW's Tau Ethereals), Captain Planet had heart, Vampire Academy had spirit, the Fifth Element had Milla Jovovich and so on. The Power of Darkness has...ok, you can probably guess, it's in the name.
Darkness Elementalists kinda save the whole thing, taking it from "I wouldn't play an Elementalist, but they might make an ok NPC I guess" to "I still wouldn't play an Elementalist, but they'd make for cool villains". They are described as being really evil (so you shouldn't want to play them) and have serious drawbacks (so you actually won't want to play them), but the drawbacks are part their appeal. You can't pick Darkness as a minor element, it has to be your major, but you can pick any two as your minor. Their magic is better than that of the normal Elementalists (admittedly, a low bar), but there are costs to using it. After the tenth time you cast the spell to increase your stealth, you permanently lose your shadow. If you use the spell that creates a zone of darkness that can drive people made, but roll badly and nobody does, you get affected yourself. That's nice and characterful, you are playing with dark forces after all.
Additionally, when you cast one of spells for your minor elements, they get an evil power up. You don't create fruit, you create nice looking, but poison fruit. You don't create rain, you create mildly acidic rain. Your magic candle creates darkness around you that increase your Evasion and Defence. Your light breeze carries voices of false rumours and conspiratorial whispering. On the other extreme, your Rank 10 summon fire monster spells doesn't just give you a monster that serves you for an hour and then disappears, it serves you for an hour and then stays for a second hour killing everyone it can.
Either way, not at all a viable PC choice, but useful as NPCs or villains. Their special equipment is something created by an 8+ Rank Darkness Elementalist in the Lost City of Nem, so they are sorta region locked in a way everyone else isn't. As an aside, 8th rank is when you start to get extra abilities in Out of the Shadows, so maybe Johnson had read that after all, and that's their ability?
Madness
After that, another 6 pages of "not very good". Never been big on having rules for mental illnesses in games, but that might just be me. Presumably it could be done well for DW (the book says "good role-playing is required"), either in the sense of dealing with the serious issue of mental health or in the sense of having fun rules to use, but that'd be something that requires more than 6 pages of barebones stuff. Either put a lot of effort in to do it right, or skip it. Hard nope from me.