There are stories of Grey Seers with huge horns who leap into the midst of battles and skewer their enemies. This spell may be the source of those stories. On casting, huge horns sprout from the Seer's head and his body grows in size by roughly 50%. The spirit of the Horned Rat flows into him. The Seer is lost in a mad frenzy and tears at all about him, friend or foe alike - such distinctions no longer have any meaning for him. He gains +3 Strength, +3 Toughness, +3 Attacks and 3D6 extra Wounds, and is subject to frenzy. He is also immune to any psychological effects and will not stop until slain.
The transformation places a strain on the Skaven's body that it is unable to sustain, and consequently there is a cumulative 5% chance each turn that the Skaven's heart will burst. When this happens, the supernatural frenzy will keep the Seer alive and fighting for another D6 rounds, after which he collapses in a dead heaap on the ground.