Note: This information was taken from TSR's original AD&D adventure, the Temple of Elemental Evil.
GARGOYLE CLOAK

When donned, this garment polymorphs the user into a gargoyle, with effect as the polymorph self spell. No command word is used. Items carried are absorbed within the gargoyle form, unusable. Immunities, flying ability, and all other gargoyle characteristics are bestowed. Armor Class is likewise altered, but is modified by the user's true Dexterity, magical effects, and protective devices worn or carried (but excluding modifications from armor and shield).

The cloak's effects may be produced up to a maximum of three times per day. The polymorph occurs within 1 segment after the cloak is donned, without the sparkling lights usually accompanying a polymorph. After the first full turn of use, there is a 10% chance per turn (cumulative) of the wearer being mentally changed into a gargoyle as well. If this occurs, the user either attacks or flies off (just as would a real gargoyle). Only a wish or death can then return the victim to normal form.

The cloak is easily damaged by any edged weapon, fire, and other hazards. If the wearer takes 20 or more points of damage from such attacks, consider the cloak destroyed. The cloak may be repaired magically (via limited wish or similar enchantment) but not by normal means.

As a final note, other cloaks of polymorphing may be found. Each is designed to change the user into one specific creature. Each confers benefits and hazards in the manner given above-a full polymorph self (lacking only the spell's curative effects), with the given chance of permanent mental change as well.