Early grave

You cast a tiny but powerful ball of lightning at your target. If your attack is successful they are knocked prone and swallowed up by the ground. They suffer 1d4 lightning damage plus 1d4 suffocation damage  plus 1d4 psych damage to start. On their next turn and all of their subsequent turns they must make a strength saving throw of dc 15 or higher or suffer 1d4 suffocation damage plus 1d4 psych damage. Allies can assist by helping dig you out on their turns instead of fighting opponents. 

Dm decides if shovels or digging tools and skill/ability scores etc add proficiency bonuses.