Source: Warhammer Armies Project: Unofficial 9th Edition

Ambush from the Worldroots
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Wood Elves invariably fight battles on the borders of woodland, where the Spellweavers can encourage the trees themselves to intervene.

A Wood Elf army can always place an additional forest no more than 12" in diameter on the battlefield. This is done during deployment, before any units are deployed, and must be placed wholly in your half of the battlefield. Declare which type of forest it is when you place it. If you cannot fit the forest on your half of the table, move other terrain features by the shortest distance necessary so that the forest can be placed. If you still cannot place the forest, or if the scenario means that the Wood Elf army does not have a table half in which to deploy, this additional forest is not used in this battle.

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