Wild is a pseudo-resource which has been introduced into the game in version 0.119.1 beta[1] in order to make multi-resource decks more viable.[2]
It is not a proper resource as scrolls can't have it in their cost, but it can be used in lieu of any of the four resources. You gain Wild by sacrificing scrolls, just like any other resource. However, the maximum of Wild you can get by sacrificing is capped to the lowest value of any other resource in your deck. In other words, if you have 2 Growth and 1 Energy, you can sacrifice for 1 Wild. If you then attempt to cast something costing 3 Growth or 2 Energy, the Wild resource will be used.
This system raises the roof for scroll costs in a secondary color, without bringing it all the way to the mono-resource level. Unlike many other systems Mojang tried, it helps "true" multi-resource decks at least as much as splash ones, and it doesn't give any boost to mono-resource decks. As an example: Sacrificing for 6 resources (2 Growth, 2 Energy, 2 Wild) lets you play scrolls of both resources costing up to 4. Previously, this value would have been 3.
In-game description[]
Wild is explained as follows in the description of Testing Grounds.
Wild is a fifth resource that is being tested in the Testing Grounds. It is a resource that can only be used with multi-resource decks. Once you have sacrificed scrolls to the Wild resource, it can be used to pay any resource costs.
There is a limit to how much Wild you can sacrifice to. You can not sacrifice to more Wild than the lowest you have of any of the regular resources included in your deck. For example; if you play a deck with Growth and Order scrolls, before you can sacrifice to your first Wild, you need at least 1 of both Growth and Order. Once you have 1 of each and a Wild, the Wild will let you play a scroll that costs 2 Growth or 2 Order, by using 1 of the regular resources and 1 Wild.