Encroaching Enfeeblement

A drop of blood is placed in the keyhole of each manacle as the magic words are spoken.  The caster then touches a creature with the manacles.

Concentration on this spell is limited to 5 minutes.  This spell creates a unique de-buff which only affects attack rolls, concentration checks, and saving throws by using a spell modified ability score.  Any movement penalties from ability score loss are halved.   This spell may only modify an ability score of caster or creature by one third.

4th - When cast the targeted creature must make an intelligence saving throw or become "afflicted".  A successful saving throw ends the spell and all spell effects.

When cast at higher levels

At the end of each turn after being afflicted, the afflicted creature may attempt an intelligence saving throw.  If successful, the "affliction" ends and there is no further effect.  If the afflicted creature fails, the DM rolls a D6 in secret to determine which ability score of the afflicted creature is immediately modified by -1.  After the first ability score reduction, saving throws are done with disadvantage +1.

5th - The D6 secret roll and ability modification is moved to the start of the creatures turn.  

6th - In addition, the caster may choose to modify a specific ability score, except intelligence.  Once the chosen ability score has been reduced by one third the DM randomly selects an ability score to modify.

7th - In addition, the ability score lost by the afflicted creature is gained by the caster; but the ability score maximum of 20 remains.  When the spell ends, ability modifiers remain for one minute.  The afflicted creature may lose an ability score even when the caster cannot increase their ability score.  Strength lost by the afflicted creature may only be strength gained by the caster.  Intelligence lost by the afflicted creature may only be intelligence gained by the caster. 

8th - In addition to previous levels, the caster may target intelligence.  Ability score changes to creature and caster remain for one hour after spell ends.

9th - In addition to previous levels, ability scores gained by the caster are retained for 8 hours.  For the caster, there is no ability score maximum or one third spell modification limit.  Reduction of afflicted creatures ability scores remains for 24 hours.  Cannot be recast at 9th level for three days.

Optional de-buff - Abilities derived from race, class, feats, or magic are not used for calculating the one third limit.  ...  Ex.  Standard humans reduce all ability scores by 1 before calculating the one third modification limit.