From the Classics: Whale Spell

The other Lappish king, Finn, turned himself into a whale. He hurled himself on top of the men who were fighting him and crushed fifteen of them to death beneath him. The dog Sel's-gift rushed at him, ripping him apart with its teeth, but as the whale opened its jaws, Hrifling's dog ran in right down to the belly, tearing at his innards, and bit away the heart. Then it ran out again and dropped down dead.

- Halfdan Eysteinsson, (Anon., 14th century AD)

The whale spell allows a magic-user to transform themselves instantaneously into a large right whale which is 50 feet long, 10 feet wide and weighs 60 tons. In water, the whale can swim at a speed of 120' per round, but on land it cannot move at all. Any creatures crushed beneath the whale take 4d10 damage, with a Reflex save for half. However, once transformed the caster is vulnerable if they are on land, and are considered wholly unarmoured for the purposes of attack rolls. Furthermore, enemies can crawl inside the whale's mouth and attack the insides for double damage. The magic-user must concentrate for three rounds in order to end the transformation; thus, the spell is often used only as a last resort.