Massmorph

When this spell is cast upon willing creatures of man-size or smaller, up to 10 such creatures per level of the caster can be magically altered to appear as trees of any sort. Thus a company of creatures can be made to appear as a copse, grove, or orchard. Furthermore, these massmorphed creatures can be passed through — and even touched — by other creatures without revealing their true nature. Note, however, that blows to the creature-tree cause damage, and blood can be seen.

Creatures to be massmorphed must be within the spell's area of effect and unwilling creatures are not affected. Affected creatures remain unmoving but aware, subject to normal sleep requirements, and able to see, hear, and feel for as long as the spell is in effect. The spell persists until the caster commands it to cease or until a dispel magic spell is cast upon the creatures. Creatures left in this state for extended periods are subject to insects, weather, disease, fire, and other natural hazards.

[Player's Handbook]

School: Alteration
Level: 4
Rarity: Common

Range: 10 yards/level
Components: V, S, M
Duration: Special
Casting Time: 4
Area of Effect: One 10-foot cube/ level
Saving Throw: None

Material Components: a handful of bark chips from the type of tree the creatures are to become (consumed)