A Mana-wyrm alone is no visible danger. A little silver slug-like thing, glimmering by lamplight and sliding along like water over a stone. Its danger is invisible, and quite potent indeed, for its food source is the arcane.
Wyrms upon the ground are an unfocuses psychic nightmare, rending the space and minds of those around it with reckless abandon. When a magic user comes close, though, the wild thrashing is replaced by a cold focus. It sees your mind and the bounty it holds. The wild strikes narrow to methodical precision. Disable food, disable the threats protecting it. If the Wyrm succeeds, it feeds.
The eye goes first, sucked clean and expelled. Then the Wyrm burrows. A horrible hermitage in bone and blood. It makes itself comfortable, then makes itself hidden. Few things down here rely on their eyes; smoothing over the host's mind is simple enough. The wyrm gorges itself for days to come, while its host finds fresh meals. More erratic as the days pass, less like the real being they once were, as wyrm-larvae eat away the grey matter. The skull bulges and, at last, splits; a new generation crawls out, wreathed in a screen of mind-tearing screams like TV static over your thoughts. The cycle begins anew.
In order, Mana-wyrms will target magic-users with innate control (if existent), magic-users, thieves, and fighters. Holy men will never be targeted, as divine powers are anathema to the beasts.