Creeping Decay

A field of decay appears in a 30 feet radius on a point you can see within range.
On the start of your subsequent turns the radius of the spell increases by 10 feet, to a maximum of 60 feet.
While the spell is active, the field is difficult terrain as the ground turns swamp-like and mushy with decay. Every living creature that starts its turn inside the field has to make a Constitution saving throw or take 4d6 necrotic damage, taking no damage a successful save. Additionally when a living creature inside the field takes damage from something that gives the creature an open wound (at the DM's discretion) the creature that 1d6 poison damage in addition to that damage, as the decay creeps inside the wound.