Living Bomb

5th-level Evocation
Casting Time: 1 action
Range: 90 feet
Components: VSM (a branding iron)
Duration: Instantaneous

A creature you can see within range must make a Constitution saving throw. On a successful save, the target takes 4d6 fire damage and the spell ends. On a failure, you successfully plant a flaming mark on the target, which detonates at the beginning of that target's next turn.

When a living bomb detonates, all creatures within 10 feet of the bomb must make a Dexterity saving throw or suffer 8d6 fire damage, or half as much on a successful saving throw. The marked creature takes damage as if it had failed the saving throw. The spell then ends.

If a creature other than the marked creature fails this saving throw, that creature is marked by a lesser Living Bomb, which detonates at the beginning of their next turn. It and creatures within 10 feet of the bomb must make a Dexterity saving throw or suffer 4d6 fire damage, or half as much on a successful saving throw.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the damage increases by 1d6 for each slot level above 5th.

Source: Homebrew