You can prepare weaker versions of spells as lower-level versions.
Benefit: Only spells that deal hit point damage and have no other effect can be cast as weak spells. A weak spell deals half as much damage as normal. Weak spells cannot also have metamagic feats added that increase their damage or save DCs. A weak spell uses up a spell slot one level lower than the spell’s actual level (minimum 1st level). Unlike most metamagic feats, a spell prepared as a weak spell calculates its save DCs using the new, lower spell level.
Example: Davor is a 9th level half–orc wizard with an 18 Intelligence. He prepares a weak fireball as a 2nd level spell. When he casts the spell, he rolls 9d6 for damage (rolling a total of 30) and it has a save DC of 16 (10 + 2 for modified spell level + 4 Intelligence modifier). Since the fireball was a weak spell he halves that damage to 15 (and targets that make successful Reflex saves take half that, or 7 points of damage). If Davor had prepared a weak still fireball as a 3rd level spell, he would still calculate its save DC as a 2nd level spell.
The Genius Guide To: Feats of Metamagic