An Eidolon of the Dark Ones is a black stone obelisk 3 strides high and 1/3 stride thick, tapering to a short point on the bottom and a long one on the top. Its four main faces are inscribed with arcane marks set into a polished surface.
An Eidolon radiates a malevolent power. Any plants within 5-10 klegs of it wither and grow pale, water sources run dry and animals sicken. Such stones apparently move randomly within their area of effect once per day at the 13th hour. An Eidolon has 8 Armour and 70 Health. A White Order Mage standing within 30 legats of an Eidolon gains 1 less corruption for each spell cast or learnt, even if that makes the spell free.