Game-Breaker: Each clan has its own way of breaking the game into little pieces in the player's hands. The usual mark of a Clan Leader is that he uses something close to the clan's best-practice build, with all clan magic on the table (though the AI never exploits infinite loops, since those are no fun for the player).
The Dark Wood clan stands out for the fact that four out of its five exclusive abilities break the game almost single-handedly, never mind when actually combined with each other. To list them:
The Curse status condition. Every hit with this condition reduces the strength of your opponent by one. There are two reasons this is broken: first, in this game damage is done by multiple fairly weak attacks, and the damage reduction applies to each hit. Second, Curse can stack, to the point where even the endgame bosses can be reduced to the same strength as the trainees fought in the beginning of the game. And Curse never wears off. While spells to remove status conditions exist, they are extremely rare (and, until endgame, impractically costly), so the number of opponents who will do so can be counted on one hand.
Increase Special. By paying a mere 20 hp, you can increase your much more important Special bar by 20 points, and this counts as a free action, meaning your opponent cannot do anything during this turn. The Special bar dictates how many of your spells you can cast. Thus, battles typically go thusly: Turn 1, increase Special. Turn 2, nail opponent with five more bolts than it is physically possible for them to have, all of which have the Curse status on them. Yes, five Curses that early on. Your opponent may now start weeping.
Silence. Bear in mind that everyone in this game is a mage, and while Silence does allow them basic attacks and wears off after one turn, you'd be surprised what you can do in that amount of time, especially when you add more Curses into the mix. Or, if their strength is already at minimum, you can cast a Combined bolt. This is a basic attack usable by anyone that turns all of your multiple attacks into a single shot, weakened by each of your opponent's attacks that hit it; not broken by itself, but when combined with their minimized strength, they will take an absurd amount of damage, with nothing they can do about it. What's the cost of the Silence spell? 20 special points.
Trade Life. Exactly What It Says on the Tin, exactly as broken as it sounds. The only limitation on this spell is that you need an almost full Special bar to cast it, but once again, Increase Special. Running low on health thanks to one Increase Special too many? Now they are. Fighting Witchthorn, or the Bonus Boss, with their absurd amount of HP? Well, wish them luck getting your new HP bar down.
While not quite as bad as Curse + Silence, the Chaos Desert clan's Burning Bolts is capable of dealing absolutely insane damage due to the way it works. As mentioned above, damage is usually done by multiple fairly weak attacks in this game, but Burning Bolts throws that right out of the window by allowing these attacks to double the effects of the last attack that hit during that turn. What this means is that if each attack normally deals 10 damage (which is pretty easily attainable with boosting items), the first attack that hits deals the normal 10 damage...then the next one deals 20...then the next one deals 30, and so on. When you add in the fact that Chaos Desert gets 10 more hits than any other clan with the exception of Great Sea (and even then, only Watervine actually uses this defensive ability), and the fact that the Chaos Desert clan enchantment reduces the opponent's Special bar to make blocking these attacks even harder...well, consider the fact that a full-power Burning Bolts deals enough damage to kill That One Bossfive times over, and the reason that enemy is That One Boss in the first place is because of their insane HP stat.
Freeze. The Ice Land mages absolutely cripple any mage who focuses on defense (except for Burning Hill, the quintessential anti-gamebreaker), because this enchantment reduces the number of defensive bolts the enemy can shoot; an ability that tends to make Great Sea and Poison Water mages cry.
Freeze Attack, an admittedly-expensive ability that allows them to cancel their opponent's attack at will.
For twenty special points, they can use Freeze Defend: a spell that allows them to attack their opponent without the possibility of any kind of defense. Though they're limited to a basic attack plus Freeze effect, this gives them ten shots of unopposed Freeze on an opponent. Which is very nasty combined with...
Freeze Death. If their opponent has 20 freezes on him, they can lay down a One-Hit KO effect. No resistance, no defense, nothing.
Burning Hill mages are refreshingly straightforward in their gamebreaking, with the additional bonus of being immune to anyone else's gamebreakers.
Increase Special. Their version uses MP, but otherwise is similar to the Dark Wood version.
Mana Shield, which allows them to take half the damage of an attack to their mana, but more importantly protects them from the special effects of their enemy's attack. No Burning Bolts from Chaos Desert, no Curse from Dark Wood.
Remove Effects, which allows them to strip out any status effects that they do take.
And finally, Mana Attack, which simply allows them to double the power of their offensive bolts and allows them to make the most powerful Combined Bolt attacks in the game.