#Dungeon23 Tomb of the Vampire Queen, Level 3, Room 2

 The bars to this cell are long gone, and it is easy to see why.  Inside is a large armadillo-like creature. Once the PCs come close to the opening, it rushes out in a frenzy of hunger.

Inside is a Magiphagous Rust Monster. It is the last offspring of one of many failed magical experiments. Many generations of these creatures have lived over the years and this is the last one. The bars to it's cell are gone, but due to a special magic placed on the other bars to the other cells it has not tried to eat those.

  

Rust Monster, Magiphagous  (Old-School Essentials)
Magical, armadillo-like creatures with long tails and two long, antennae-like feelers. Feed on the remains of magical metals.
AC 2 [17], HD 7 (31hp), Att 1 × feeler (rusting), THAC0 13 [+6], MV 120’ (40’), SV D11 W12 P13 B14 S15 (5), ML 9, AL Neutral, XP 175, NA 1d4 (1d4), TT None

• Rusting: Magical metal that touches a rust monster (e.g. weapons that hit it, or armour struck by a feeler) crumbles instantly to rust.  Each time a magic item is affected, it loses one “plus”.  Each item gets a base saving throw of 13, plus any "pluses" the magical item has.
• Mundane damage immunity: Can only be harmed by magical attacks.
• Smell metal: Attracted by the scent of magical metals such as weapons, armors and artifacts.

Appearing as a paler and larger version of the rust monster, these creatures are otherwise exactly the same as rust monsters, save for one detail.   These creatures only eat magical metals.