Amber

 is a special resource for the Wood Elves race. Amber works differently in Total War: Warhammer, compared to game II and III.

How it works (Warhammer II)
Amber was significantly stripped down with The Twisted and the Twilight DLC and Wood Elves overhaul. Amber is now only gained from fully healing a magical forest settlement. It is now only spent on select technologies that give powerful buffs. Thus, it impossible to have negative Amber.

How it works (Warhammer I)
The total amount of amber you can access is seen at the top center of the campaign screen. Amber is never expended like money, just temporarily used or "taken up". If amber is no longer being used then it goes back into the total pool. For instance if you take a settlement then 1 amber is added, and if you lose the settlement then 1 is taken away. Likewise, if a unit is consuming 1 amber and then it is desroyed or disbanded, 1 amber goes back into the total.

Having a negative total amber (ie: you are using more than you are producing) will cause penalties for your faction.

Confederating other Wood Elves
After confederating another Wood Elf faction, you can end up getting some negative amber totals from them, as the AI seems to be able to cheat and go below 0 without first acquiring any.

Negative Amber Penalties
When you have a negative total amber you get the following penalties:
 * between -1 and -9: -5% Casualty replenishment rate
 * below -9: -5 to public order, no unit replenishment
 * Past -15: attrition which overrides replenishment even in friendly territories

Acquiring Amber

 * Each settlement (except the Oak of Ages) you control generates amber.
 * Minor settlements: 1
 * Major settlements: 2
 * Each military alliance with another faction generates 1 amber per settlement they control.

Uses

 * Amber is required to upgrade the Oak of Ages, and thus win the campaign.
 * A total of 30 amber is required for a fully upgraded Oak.
 * The Wood Elves faction requires amber to recruit elite forest spirit and monster units:
 * Treemen
 * Forest Dragons
 * Great Eagles
 * Tree Kin
 * Hawk Riders
 * The Argwylon faction requires amber to recruit elite elven units:
 * Wardancers
 * Wardancers (Asrai Spears)
 * Wildwood Rangers
 * Sisters of the Thorn
 * Waywatchers
 * Several Wood Elves technologies use amber once researched. See that page for details.