A Troll's digestive fluid is the most corrosive thing known to science... and about the most revolting too. Trolls are able to vomit their stomach contents upon their enemies if they wish. A unit of Trolls can make Vomit Attacks instead of ordinary attacks during close combat.
Source: Warhammer Fantasy: 6th Edition
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Instead of attacking normally the whole unit may elect to vomit on the enemy. As each Troll can make only 1 Vomit Attack at a time, as opposed to 3 ordinary Attacks, this means the unit gets fewer Attacks, although these always hit. Each model inflicts one automatic Strength 5 hit with no Armour saves allowed.