When you perform a controlled polymorph, you are only able to turn into monsters which have HD equal to or lower than your level. If you are using a mask, or if you are polymorphing randomly, it treats you as being 10 levels higher than your actual one.

Lycanthropic and vampire shapeshifting polymorphs are exempt from this restriction.

Randomly generated statues and figurines (and masks in variants that have them) should use a higher effective difficulty when choosing the associated monster species, because using low-level monsters is boring and typically useless to the hero.

This should probably also apply to statue traps, since they are not very threatening at the moment.

An artifact mask Mask of Many Faces, neutrally aligned. When invoked, you are prompted to target an adjacent creature. If that creature fails a monster magic resistance check, or a d(its maxHP) roll is lower than its current HP, it is instantly killed, dropping its possessions but no corpse or other special drops, and the mask changes to that creature’s form.

Subsequently donning the mask, or having it worn when invoking it, merges it into your skin and polymorphs you into that monster’s form, starting a timer (not the usual polymorph timer) after which the mask will turn blank and the polymorph ends. Lower level monsters than the player will produce a longer timer, and higher level monsters and uniques will produce a shorter timer. You can also invoke it again while wearing it to end the polymorph early.

When choosing the monster type for a mask, use a higher difficulty threshold than it would take to generate monsters; otherwise masks only generate with level-appropriate monsters which the player probably doesn’t want to turn into.