Planar Collapse

At the beginning of your upkeep, if there are four or more creatures on the battlefield, sacrifice this enchantment and destroy all creatures. They can't be regenerated.

Light-Paws, Emperor's Voice

Whenever an Aura you control enters, if you cast it, you may search your library for an Aura card with mana value less than or equal to that Aura and with a different name than each Aura you control, put that card onto the battlefield attached to Light-Paws, then shuffle.