The newbies just faced one of these in a fey dungeon within a semi-sentient plant, guarding a rather nasty trap that they thought was their ultimate goal...
Created by certain arch-fey or potent wielders of primal magic, these things are not so much built as grown. Half again as tall as the average human, they are willowy, slender things. Most are vaguely humanoid in form, though their bodies consist of thickly intertwined thorns of verdant, vivid green, which coil tightly together before emerging from their surface in a brutal mantle.
When activated, more thorns - usually crimson and dagger straight, emerge from between the curving mass of the thing's component growths, creating a deadly barrier to those who get careless when battling the brute.
Barely sentient, these things obey the commands given to them by their masters in a straight forward and literal sense. They do not flee, and fight until destroyed.
(Special Note: The software I use to generate these blocks does not add the modifier to the stat mods for 1/2 the monster's level. Add +1 to those listed above; i.e. the Dex mod is +3 not +2)