It attacked us... Gods, I dunno what it were, but 'twere at least four spans high!
-- Velen peasant woman
Velen's woods and bogs brim with monsters. This fact is well known, and thus when the war drove refugees into this hostile land, certain of their number thought to avoid the dangers of the lowlands by hiding in an abandoned mine. They were in for an unfortunate surprise. The shaft they chose for their sanctuary was inhabited by a poisonous arachas - a merciless and bloodthirsty monster.
The witcher knew he was in for a tough fight. The arachas had dwelled in the mine for years and had grown large and particularly strong and resilient in that time. Like other members of its vile species, it would be dangerous both at close quarters and at a distance. What's more, it was almost certainly venomous - he would need to imbibe Swallow or Golden Oriole (or both) before attacking if he was to stand a chance of surviving. As a small consolation, the witcher knew the monster would be vulnerable to his Signs - especially Igni, Aard and Yrden - as well as to the Northern Wind bomb.
The witcher arrived at the cave too late to save the refugees - but he could still avenge their fate. He slew the powerful arachas and destroyed the eggs bearing its vile offspring.