Blood of the slain

You spend one hour weaving this spell into the structure of a weapon (This spell is cast as a ritual).

When you kill a creature what has blood with this weapon the weapon deals more damage dice + your spell casting mod on all attacks you make until the spell ends. If the creatures challenge rating use under your easy xp threshold nothing happens. If the creature is in your easy threshold the damage increase is 1d6. If its in your medium threshold the increase is 1d8. If its a hard encounter the increases is 1d10. For a deadly encounter the increase is 1d12. A weapon may not have more than 4 extra dice at once. 

The number of dice on attacks and number of extra dice on the weapon is increases by 1 for every spell slot level above 5th.