Add a bit to the monst struct that tracks whether the monster is hiding on the ceiling or not. (Only valid for things like wumpuses, piercers, and perhaps bats but not actually since bats don’t roost on the ceiling when inactive.)

Then, when polymorphed into a wumpus or piercer, you can use < when not on stairs to climb onto the ceiling and > to climb back onto the floor, but only when adjacent to a wall.

Piercers can actively perform their falling attack from an adjacent space, rather than passively waiting for the player to step onto their space. After falling, they can only make a weak claw attack against the player, and instead of engaging will try to pathfind to a wall. Once they get adjacent to a wall, they return to the ceiling and become able to use their falling attack again.

A further change to make them more interesting than just discovering them and hitting them would be to make them too high to hit in melee while on the ceiling, unless the player is in a Huge form or larger.

Increase piercer damage by a lot, make the piercer hit ignore AC so that it hits most of the time, and make helms not count for much against a falling piercer.

Rock piercers instakill stone golems, glass piercers instakill glass golems and iron piercers instakill iron golems.