I've not used them since...until....soon....maybe.
So, here they are, the metal ignoring, yokel faced elementals of fire and earth, ready to tear it up in your 5e game. As always, enjoy!
Lava Children – Medium Elemental (Earth, Fire)
A.C. 14 (Natural)
Hpts: 58 (9d8+18) [27 - 90]
Proficiency Bonus: +2
Str: 13 (+1) Dex: 11 (+0) Con: 15 (+2) Int: 10 (+0) Wis: 11 (+0) Cha: 11 (+0)
Saving Throws: Constitution +4
Damage Immunity: Fire, Poison; Metal Immunity
Condition Immunity: Poisoned, Petrified
Damage Vulnerability: Cold
Senses: Tremorsense 30'; Passive Perception: 10
Language: Elemental Native
CR: 4 (1,100 xp)
Metal Immunity: Metal simply does not exists for these creatures, and passes through them as if they were made from air. Similarly, they can move unhindered through all kinds of metals. Attacks made by metal weapons inflict no damage on them, and metal armours and shields do not offer individuals protection against their attacks.
Multiattack: The Lava Child makes two punch attacks
Punch: Melee Weapon Attack; Reach 5'; One target; +3 to hit; Hit: 8 (2d6+1) bludgeoning damage +7 (2d6) Fire damage. Hit 5+: Target must make a D.C. 12 Dexterity saving throw or catch on fire. Whilst on fire, they suffer 7 (2d6) Fire damage at the start of each subsequent turn. An adjacent individual or the target themselves may spend an action to allow them to repeat the saving throw to end this effect.
Can you guess which is from the original Fiend Folio?