Each character has special flags that are stored in an 8 bit value. They grant some immunity and hence some are used in a way that matches elements of specific monsters, I guess it can also denote some monster element.
| Bit | Property |
|---|---|
| 0 | Unknown. Only used by the guard demon who is immune to all kind of damage and spells, no effect found. |
| 1 | Psychic element, immune to MonsterKnowledge, Fear, Sleep, Irritation and Madness |
| 2 | Ghost element, immune to DissolveVictim, Blindness, GhostWeapon, MagicProjectile and MagicArrows. |
| 3 | Undead element (all undead), immune to LP-Stealer. |
| 4 | Stone/earth element (golems), immune to Poison, Paralyze, Disease and LP-Stealer. |
| 5 | Wind element (Gargoyle and imp but not minor demon), no effect found. |
| 6 | Fire element (Demons and fire monsters), no effect found. |
| 7 | Water element (only the pond lizard), no effect found. |
First I thought that the elements would give several immunities against element spells but this doesn't seem to be the case at all.
Guard demon has a value of 0xff so he has all 8 elements and their immunities. He is the only character which uses the first 3 elemens.
Dark mage (Tar the dark) has a value of 0xf0. So this means he has all the 4 major elements: earth, wind, fire and water.
A petrified enemy gains total immunity against all damage. You cannot attack and damage dealing spells won't do anything. But I guess you can use DissolveVictim on them.
Ironically petrify works on golems which have earth element.
The pond lizard is the only pure water element monster but I didn't find any effect or immunity.