Massmorph (from Cook)
4th Level MU/Elf
In a nutshell, it makes up to 100 man-sized creatures within a 240' circle appear as trees (orchard/forest). It lasts until dispelled or dismissed by the caster. Even someone moving through the area does not reveal the disguised subjects. It is an ideal spell for ambushes and disguising troop placement.
I can see elves making great use of this spell, hiding a company of warriors in the middle of their own forest, letting an invader walk right past them until they were surrounded. In fact, this could be a devious little trick a DM could play on a lower level party that wanders into a sylvan wood. Not necessarily for a TPK, but definitely a possible "change your armor" moment.