Walking Bomb

You nominate a fleshy vessel to be your living bomb. Does not effect constructs. Target must pass a Constitution saving throw against the caster spell DC. on a failed saved they are infected for 3 rounds. On the start of each of their turns while infected they take 1D12 necrotic damage. If the target dies before the spell ends; they explode and creatures within 15 ft must make a constitution saving throw with advantage or also be affected (triggers only once when cast at second level). For each level upcast add  one additional D12 to the damage dice each turn. When cast at 3rd level The chain reaction constitution save is made normally. at 5th level or higher the chain reaction can continue beyond the first stage. at 7th level or higher they Constitution check is made with disadvantage and the explosion radius is increased to 30ft