You can prepare less effective versions of spells as lower-level versions.
Benefit: Only spells that allow a saving throw for half effect or to negate the spell’s effect can be cast as languid spells. A languid spell does not add its spell level into calculations to determine its save DC. A languid spell uses up a spell slot two levels lower than the spell’s actual level (minimum 1st level).
Example: Xasha is a 9th level bard with an 18 Charisma. She prepares a languid mass charm monster as a 4th level spell. When she casts the spell, she calculates its Will save without including the spell level, resulting in a save DC of 14 (10 +4 Charisma modifier).