If you want to make treants a big part of your campaign, get this book. It contains all you need. If you like Mongoose's "Slayer's Guide" series (which I do), this book is on par with them. Illustration-wise Goodman Games might not have the same capacities, but content wise, this book is top notch.
Now, because you decided to read this review, I will keep going and delve deeper in the book's content to give you an adequate overview.
This little book gives a thorough account of the ecological existence of treants. Page after page describe the thousand year lifespan of the treants, their society, reproductive cycles, politics, attitudes towards the "axe-wielding problem" (i.e. the young races - humans, dwarves, goblinoids, etc...). Many aspects of the treant's culture are discussed at exhaustive length. A description of what happens to treants who "fall into shadow" or become evil suggests some good ways to use treants as antagonists.
The treants' evolution is mapped through a HD progression and a few treant specific classes and prestige classes. While the HD progression as opposed to character class is a bit fuzzy in terms of game mechanics, any DM worth is salt will work out this minor potential incongruity. The HD progression of treants map their existence from 1HD sapling to the 7HD Huge creatures described in the Monster Manual, all the way to Gargantuan 20HD treants, with appropriate ability adjustment and special abilities.
Three 10 level and one 5 level prestige classes available only to treants are proposed. The 5 level class is the firesworn, a treant whose forest was ravaged by fire and who vows to have vengeance on those who are responsible. The other three classes are the treant equivalent of the druid, the bard and the ranger.
A new kind of magic items, living magic items, lets treants enchant animals and plants, such as a boar with a shocking gore attack and plants that grow potion-fruits. Feats improve on the treants' natural abilities and special attacks such as trample and damage to objects. There are also five new spells available to the treant spellcasting classes.
Four templates that can be applied to treants further expands the palette of the DM. Two of these are undead templates; treants destroyed by fire (blasted treant) and treants fallen deeply into the shadow (hollow treant). The deep treant is a mutation adapted for underground life, and the forsaken treant is a raging plant of destruction and fear.
The book concludes with the description of two treants and their lairs, using the classes and templates described above. Adventure hooks are suggested. Like I said earlier, this very well made book contains everything you need to exploit treants very thoroughly.
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