The special chameleon-like qualities of the Mandrakes allow them to blend into the background, even when standing in open terrain. Mandrakes always count as being in cover, giving them a 5+ cover save. This counts as an assault as well, so unless the attackers are armed with frag grenades, the Mandrakes will always strike first. If they are in cover, they gain a +2 bonus to the normal cover save. If attacked by something that ignores cover their normal armor save of 5+ applied.
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