There are six damage types with Elemental vulnerabilities to them: Fire, Shock, Cold, Poison, Iron, and Salt.

Whenever a unit is hit by a damage type they're vulnerable to, if it's one of these four types, the initial damage (what goes into the Damage DRN roll) is either doubled or increased by the point value of the vulnerability; whichever adds less.

For example, say a Jotun Bondi with Shock Vulnerability (5) is hit with a Thunder Strike. The damage of the main part of the Thunder Strike is usually 26; since this is shock damage, however, it's increased to 31. This alone is just enough to kill them in one hit. The secondary damage of the spell is usually 2; this is also shock damage and, since 2 is less than 5, the damage is only doubled (to 4).

Within the game's code, there is actually no such thing as an Elemental Vulnerability. It is recorded as a negative Elemental Resistance to the same damage type. Therefore, adding 10 Elemental Resistance to 5 Elemental Vulnerability results in 5 Elemental Resistance.

An Iron Vulnerability adds to the damage taken from iron weapons and certain iron attack spells, as long as the attack does at least one point of damage without it. It isn't capped at double damage like the previous four. Blade Wind and Iron Darts are iron attack spells, for instance. The iron in arrowheads is not considered to be enough iron to trigger the vulnerability.

Note that iron doesn't have a damage icon; for some weapon types (where it isn't obvious), you may need to click on them and see if they are made of iron. However, not many magic weapons are made of iron.

A Salt Vulnerability changes the damage cap on Enchanted Salt. For those without Salt Vulnerability, Enchanted Salt's damage is capped at zero after the Damage & Protection DRN rolls; thus, this trait literally is a measure of how vulnerable one is to Salt. Even Salt that doesn't do damage can Stun people, though.