Arachnid Eradication

Instantly kills all arachnoid creatures with less than 250 HP within a thirty foot sphere centered on the point where the spell is cast. Includes all common spiders, dryders, and anything with arachnoid characteristics. In the event of confusion, the DM has final say as to the nature of "arachnoid characteristics." As you use higher level spell slots than level 3 to cast this spell, the maximum amount of HP increases by 50, up to a maximum of 550 HP total at level 9. 

"Spider's are scary, let's kill them!" - Someone, probably